Welding and allied processes — Procedure for determining the hydrogen content in arc weld metal

(IIW-Document: II-1567-05; Thomas Kannengiesser )

Annex A: Determination of diffusible hydrogen in higher and high alloyed steel and weld metal

Annex B: Determination of diffusible hydrogen in nonferrous and weld metal

(This could be incorporated into the Draft Standard as Annexes. Only some remarks would be required in this case referring to the specific material-dependent features of the welding process and of the weld metal production (Sections 3.1 and 3.2) that have to be considered.)

1 Scope

This International Standard specifies the sampling and analytical procedure for the determination of diffusible and residual hydrogen in ferritic weld metal arising from the welding of ferritic steel using arc welding processes with filler metal.

With application of the primary method, diffusible hydrogen is collected at room temperature (static method) via an appropriate hydrogen sensor or chemical substrate (e.g. zeolite, adsorbent, molecular sieve). The hydrogen measurement can be carried out directly or via an chemical substrate which is subsequently used for hydrogen measurement.

The decisive advantage of the static method is that only purely diffusible hydrogen escapes when hydrogen is collected at room temperature. Material-dependent activation of potential trapped hydrogen sources shall be avoided by the static method.

The static procedure will be specified as primary method which can be used for calibration of other methods. An off-spec method calibrated using the primary method can be employed for usual testing. The primary method specified in this Standard shall be applied in cases of dispute.

With application of the dynamic method the hydrogen measurement can be carried out at elevated temperatures (carrier gas hot extraction/melt extraction), of which the temperature level is dependent on the material or appropriate chemical substrate.

The measurement of the total hydrogen content (diffusible plus trapped) in ferritic weld metal is be carried out directly on the weld metal specimen at elevated temperatures immediately by carrier gas hot extraction/melt extraction. Hydrogen extraction is accomplished in this procedure within a much shorter period of time (rapid method). This measuring method generally yields higher hydrogen contents than the above mentioned combined primary method, since further hydrogen traps can be activated depending on the degassing temperature and on the available weld metal. Specification of an elevated degassing temperature for the diffusible hydrogen portion is therefore only possible with knowledge of the activation temperature of the hydrogen traps depending on the material to be investigated. The degassing temperature recommended in this standard for the diffusible hydrogen content is based on experience gained with ferritic weld metals and may deviate from that in an actual case. For exact determination of the degassing temperature for purely diffusible hydrogen, driving of measuring ramps at different degassing temperatures is additionally recommended in the concrete case.

Static and dynamic methods can be combined. The idea behind this combined procedure is to ensure direct comparability with results of outdated primary methods (mercury method) which will be superseded due to their health hazards.

Provided that the weld specimen size is maintained within limits dictated by the size of the test block, variations in welding parameters are permissible in order to investigate the effect of such variables on the weld hydrogen content. .

2 Normative reference

The following normative document contains provisions which, through reference in this text, constitute provisions of this International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent edition of the normative document indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards.

ISO 14175, Welding consumables — Shielding gases for arc welding and cutting.

3 Test procedures

3.1 Production of weld specimens

3.1.1 Principle

The welding process to be tested is used to deposit a single weld bead which is rapidly quenched and subsequently stored at –78 °C or lower until required for preparation and analysis.

3.1.2 Welding fixture

A copper welding jig, which may be water cooled, is shown in Figure 1. It shall be applied to provide uniform test pieces for shielded metal arc, gas metal arc, flux cored arc and submerged arc welding for heat inputs up to about 3 kJ/mm. It is designed to promote the proper alignment and clamping of the test piece assembly by means of the single clamping unit which is used with a ring spanner or other suitable means. See 3.1.4 for evidence of proper alignment and clamping. A welding jig without water cooling may be used as long as the same dimensions are retained and as long as the temperature is controlled in the manner described in 3.1.4 below.

Dimensions in millimetres

Key

1

Copper block

2

Test piece assembly

3

Copper foil

4

M12 bolt

NOTE Water cooling channels may be used.

NOTE: The run off bead length shall be such that the trailing end of the crater is on the run off piece but within 15 mm of the test piece; a Dimension X ≤ 15 mm.

Figure 1 — Welding fixture and test piece assembly for weld deposits

3.1.3 Test piece assemblies

The test piece assembly shall be prepared from a plain carbon non-rimming steel with a carbon content of not more than 0.18 % and sulfur content of not more than 0.02 %. The test assembly shall be made according to the dimensions shown in Figure 2, with a tolerance of ± 0.25 mm on all dimensions except the length of the run-on and run-off pieces. The lengths shown in Figure 3 for the run-on and the run-off piece represent minimum values.

Note 1: Specimen geometry/ specimen volume shall assure in particular also the exact ascertainment of low hydrogen contents (< 5 ppm in ferritic weld metal)

Note 2: comparable to the specimen geometry according to ANSI/AWS A4.3-93 [5]

Dimensions in millimetres

Test assembly

la = lb

lc

e

t

Figure 1

35

80

25

12

A/B

Run-on / Run-off test piece.

C

Centre test piece.

NOTE The centre test piece has the same dimensions in all the three cases.

Figure 2 — Dimensions of the weld test assembly

All surfaces shall be finished at right angles to ensure good contact between adjacent pieces during the welding operation. Each test piece assembly may be finished with one operation on a surface grinder so as to ensure a uniform width, or closer dimensional control may be exercised to obtain proper clamping. See 3.1.4 for evidence of proper clamping.

Prepare three or more sets of test pieces and number them by engraving or stamping the opposite side to that to be used for welding. Number and degrease each centre test piece in each set. Determine the weight of each centre test piece (m1) to the nearest 0.01 g. Degas the centre test pieces in a vacuum, or dry inert carrier gas, at (650 °C ± 10 °C) for 1 h and cool in a vacuum or inert carrier gas prior to weighing. It is permissible to degas the steel from which the test piece assembly is made prior to machining operations, in which case it is not necessary to degas the centre piece after machining. It is also permissible to degas in air when this is followed by complete removal of surface oxide by grit blasting with a clean, dry abrasive. In case of dispute, the run-on and run-off pieces shall also be degassed.

For all welding processes the test piece assembly is clamped in the welding fixture using annealed copper foil as shown in Figure 1. The annealed copper foil may be used to prevent erosion of the fixture. The foil may be annealed repeatedly and quenched in water after each annealing. Oxide scale after annealing is removed by pickling with dilute nitric acid (10 %) followed by washing with distilled water and drying.

3.1.4 Welding and test piece storage

The temperature of the welding jig before each weld is made shall be ambient or not more than 25 °C above ambient. If difficulty is caused by condensation of water on the jig and test piece assembly, it will be necessary to use cooling water thermostatically controlled to ambient temperature or as much as 25 °C higher. Using the welding process as specified in 3.2, and parameters appropriate to the type of investigation, make a single weld bead on the test piece assembly that is clamped in the welding jig as shown in Figure 1.

a) Welding shall be initiated on the run-on piece at a point sufficiently distant from the centre test piece such that a stable arc and a stable deposit shape are achieved before reaching the centre test piece.
b) Welding shall be terminated with the trailing edge of the crater within 15 mm of the centre piece.
c) After extinction of the arc, and without any delay, the clamp shall be released and the test piece assembly removed and quenched as rapidly as possible to below room temperature in stirred iced water and then transferred to a low-temperature bath saturated with solid carbon dioxide, or to liquid nitrogen.
d) Once chilled, the underside of the central test piece shall be examined to assess the uniformity and extent of heat tinting. Properly aligned and clamped test assemblies shall show parallel and uniform heat tinting of the underside of the central test piece, and dark oxidation shall not extend to the edges of the underside of the central test piece.
e) Slag shall be removed, the run-on and run-off pieces broken off and the centre piece returned to cold storage. The centre pieces may be stored at -78 °C in the solid carbon dioxide bath for a period of up to three days, or at -196 °C in liquid nitrogen for a number of weeks ( 3 weeks at maximum) if necessary, before analysis.
f) For purposes of classifying welding consumables, during welding of the test assembly, the ambient absolute humidity shall be at least 3 g of water vapour per 1 000 g of dry air. (This corresponds to 20 °C and 20 % relative humidity.) When the absolute humidity, measured using a sling hygrometer or other calibrated device, equals or exceeds this condition, the test shall be acceptable as demonstrating compliance with the requirements of this International Standard provided that the actual test results satisfy the diffusible hydrogen requirements of the applicable consumable classification standard.

3.1.5 Recording of data

All welding details such as current, voltage, travel speed, filler metal type and composition, etc. shall be recorded on the appropriate weld data sheet as given in 3.2. It is particularly important to record atmospheric temperature and humidity at the welding station. All these data are reported with the analytical results.

1

2

3

1

2

3

Average

3.2 Welding procedures for the production of weld specimens

The welding process under investigation shall have its operating parameters defined so as to permit the production of a single weld bead on the test piece assembly described in 3.1.

3.2.1 to 3.2.3 describe the procedures for different welding processes.

3.2.1 Metal arc welding

3.2.1.1 Electrodes

The covered electrode to be tested shall be used in one of the following ways:

a) For the purposes of classification, the electrode and the method of deposition of the weld shall be as specified in the standard with which the electrode complies.

b) For the purposes of investigation, the electrode and welding parameters shall be those given in the specific

welding procedure. If no procedure has been given, then a current which is 90 % of the maximum suggested

by the manufacturer shall be used.

When a predrying treatment is specified, the time and temperature specified by the consumable's manufacturer shall be used. If a range is given by the manufacturer, e.g. 300 °C to 350 °C, the average shall be used.

Electrodes with cracked or broken coatings shall not be used and electrodes to be tested in the as-received condition shall be taken from a freshly opened undamaged packet. During any drying treatment the electrodes shall not touch each other or the side of the oven. During any drying operation a calibrated oven shall be used and the electrodes shall spend the full specified time at the drying temperature. Only electrodes under test shall be placed in the oven during this time. When the drying operation is complete, the electrode shall be cooled to ambient temperature in a container, e.g., a dried borosilicate glass tube sealed with a rubber bung. The electrode shall be used as soon as possible after it reaches ambient temperature. Any electrodes removed from the drying oven and not then used shall not be redried and subsequently used for the test.

When electrodes are to be tested in the as-received condition from a hermetically sealed container, the electrodes shall be protected from moisture pickup once the seal is broken, until each can be welded. Some sealed containers are resealable. In such a case, each test electrode can be withdrawn individually and the container resealed while the withdrawn electrode is welded. If the container is not resealable, then all of the test electrodes shall be withdrawn when the seal is broken, and each electrode shall be individually placed in a dried borosilicate glass tube sealed with a rubber bung until the electrode is to be used for test.

3.2.1.2 Making the test welds

A copper fixture, such as that shown in Figure 1, shall be used for the alignment and clamping of the test piece assembly, which uses a 25 mm × 12 mm × 80 mm length centre test piece.

If the classification standard is silent on this matter, the following shall apply. The classification of covered electrodes is carried out using 4 mm diameter electrodes. In this case the welding current shall be 15 A less than the maximum or 90 % of the maximum stated by the manufacturer, being controlled within a tolerance of ± 10 A. The speed of welding shall be adjusted to produce (10 g ± 1.5 g) of deposit on the centre test piece, which is usually accomplished by an electrode consumption of between 1.2 cm and 1.3 cm per cm of weld.

Three or more test welds shall be made on different test piece assemblies using a different electrode for each weld. The deposit shall be made, without weaving, along the centre line of the test piece assembly which is usually aligned as shown in Figure 1. The dimensions of the run-on and run-off pieces are 15 mm × 12 mm × 35 mm. No burning-off prior to testing shall be allowed. The run-on deposit length shall not exceed 15 mm. The time spent in deposition shall be noted. The trailing end of the crater shall be on the run-off piece but no further than 15 mm from the central test piece. The unused portion of electrode shall be retained for measurement. The method of using the welding fixture is described in 3.1.4. When welding is completed, the weld specimen shall be quenched and may be stored as described in 3.1.4., after which it shall be cleaned and analysed for hydrogen content as described in 3.3.1.2 to 3.3.1.4.

At the time of welding, due to the influence of atmospheric moisture on the test results, for purposes of classifying covered electrodes, the arc length shall be maintained as short as possible consistent with maintaining a steady arc. For all purposes, the details listed in 3.2.1.3 shall be recorded.

3.2.1.3 Recording of welding data and results report form

The following report sheet gives full details of all the test variables which pertain to the test results.

Report form (diffusible hydrogen, metal arc)

Investigating laboratory: Date: Investigator's name: Make of electrode: Batch No.: Type of electrode: Electrode designation: Diameter of electrode (mm): Overall length of electrode (mm): Drying treatment: …..°C for ..…h Electrode polarity (d.c. +ve, d.c. -ve or a.c.): Relative humidity …..(%) and temperature …..(°C) at the welding station during welding Approximate evolution temperature: …..°C Hydrogen collection time: …..d; …..h

Number of test piece: Voltage, V; a.c. or d.c.: Current, A: type of meter: Welding time, s: Weld length, mm: Heat input, kJ/mm: Electrode length used, mm: Run-on length, mm: Mass of deposited metal on test piece, g: Test piece to crater distance, mm:

Diffusible hydrogen (a) HD, ml/100 g of deposited metal: (b) HF, ppm of fused metal:

Other test details not included above:

3.2.2 Submerged arc welding

3.2.2.1 Electrode wire

The diameter of the consumable wire used in the submerged arc welding process is linked to the current used and to the size of the weld bead. In general this is a high current process with consequently large weld beads.

The consumable solid or cored wire to be tested shall be used in one of the following ways:

a) For purposes of classification, the welding parameters shall be the same as those used in the preparation of

the all-weld-metal test assembly for mechanical property determination, with travel speed adjusted to provide a deposit weight on the centre test piece of 10 g ± 1.5 g.

b) For the purposes of investigation, the electrode wire and welding parameters shall be those given in the specific welding procedure. The use of a solid wire which has been degassed in a vacuum or inert gas at 650 °C for 1 h facilitates the investigation of the effect of welding parameters, and type of flux and its drying procedure, upon the hydrogen content of the weld.

The arc energy for making the weld is restricted.

3.2.2.2 Flux

When drying is required, the flux shall be dried in one of the following ways:

a) for the purposes of classification, in accordance with the requirements of the standard with which the flux complies;
b) b) for the purposes of investigation, in accordance with the manufacturer's recommendations.

At least 1 kg of flux is required for three welds. Drying shall be done in an open container placed in a calibrated drying oven set at the correct temperature. The maximum flux depth shall be 15 mm.

The flux shall spend the full specified time at the drying temperature and other fluxes shall not be placed in the oven during this time. When the drying treatment is complete, the flux shall be cooled to ambient temperature in a sealed container, where it shall remain until required for use. Used flux shall not be re-cycled.

3.2.2.3 Making the test welds

A copper fixture, which may be water-cooled, such as that shown in Figure 2, shall be used for the alignment and clamping of the test piece assembly. The spring-loaded lever clamp ensures that the applied pressure is uniformly tight from test to test to ensure good thermal contact. Water cooling is an essential aid to the rapid through-put of test pieces.

The centre piece remains the same size as described in 3.1, but is aligned with longer run-on and run-off pieces (135 mm) as shown in Figure 2. The preparation, degassing and use of the test piece assembly is described in 3.1. The flux is kept at a predetermined constant depth of 30 mm by levelling off along the top of the copper foil inserts shown in Figure 4. If a different flux depth is specified by the flux manufacturer, then the dimension of the copper foil shall be modified in order to achieve the specified flux depth. At the end of the copper foil there shall be a suitable piece of copper foil to contain the flux.

Three or more test welds shall be made on different test piece assemblies. The deposit shall be along the centre line of the test piece assembly. The time spent in deposition shall be noted. The trailing end of the crater shall be on the run-off piece but no further than 15 mm from the central test piece. No length for the run-on portion of the weld deposit is specified, but the length shall be sufficient to achieve arc and deposit stability before reaching the central test piece. The welding fixture is used as described in 3.1.4.

The range of consumable wire diameters, and therefore the range of currents and welding traverse speeds, enables variations in welding parameters to be made within the maximum heat input. Generally, the values chosen shall be compatible with the welding parameters recommended for a particular wire diameter.

The welding current, polarity, voltage, time, weld length, wire feed and electrode extension (stickout), ambient temperature and humidity shall also be noted.

After extinction of the arc and without any delay, the test piece assembly shall be released from the fixture and the test piece quenched, cleaned and stored as described in 3.1.4.

For all purposes, the details listed in the report form under 3.2.2.4 shall be listed.

Key

1 1 mm copper foil, 40 mm × 300 mm

2 Test piece assembly

3 Welding fixture

1

2

3

1

2

3

Average

Figure 4 — Use of copper foil to maintain constant flux depth

3.2.2.4 Recording of welding data and results report form

The following report sheet gives full details of all the test variables which pertain to the test results.

Report form (diffusible hydrogen, submerged arc)

Investigating laboratory: Date: Investigator's name: Electrode diameter, mm: Electrode designation Make of electrode: Batch No.: Type of flux: Flux maker: Batch No.: Flux drying temperature and time: …..°C for ..…h Electrode polarity (d.c. +ve, d.c. -ve or a.c.): Relative humidity …..(%) and temperature …..(°C) at the welding station during welding Approximate evolution temperature: …..°C Hydrogen collection time: …..d; …..h

Number of test piece: Voltage, V; a.c. or d.c.: Current, A: type of meter: Welding time, s: Weld length, mm: Welding speed, mm/s: Heat input, kJ/mm: Wire feed speed, mm/s: Electrode extension, mm: Run-on length, mm: Mass of deposited metal on test piece, g: Test piece to crater distance, mm:

Diffusible hydrogen (a) HD, ml/100 g of deposited metal: (b) HF, ppm of fused metal:

Other test details not included above:

3.2.3 Tubular cored electrode with or without gas shield and wire electrode with gas shield

3.2.3.1 Filler material

The filler material to be tested shall be used in one of the following ways:

a) For purposes of classification, the welding parameters shall be the same as those used in the preparation of the all-weld-metal test assembly for mechanical property determination, with travel speed adjusted to obtain a deposit weight on the centre test piece of (10 g ± 1.5 g). It is well established that diffusible hydrogen results from tubular cored electrodes are strongly affected by the electrode extension. Care shall be taken that the electrode extension used for the diffusible hydrogen test is the same as that used in preparing the all-weld-metal test coupon for mechanical property determination.

b) For investigation purposes, the filler material and welding parameters shall be those given in the specific welding procedure. If a current range is given by the manufacturer, then the average shall be used. When an unused reel of filler material is being tested, the complete outer layer shall be discarded.

3.2.3.2 Shielding gas

The shielding gas shall be of a welding grade as specified in ISO 14175. The shielding gas used and gas flow shall be according to the manufacturer's recommendations. Details of the shielding gas composition and flow shall be recorded on the report form. For investigation purposes it may sometimes be necessary to dry the shielding gas in order to remove moisture. If this is done, then the moisture content of the gas shall be measured and reported.

3.2.3.3 Making the test welds

A copper fixture such as shown in Figure 1 shall be used for the alignment and clamping of the test piece assembly.

The copper fixture shown in Figure 1 may incorporate water cooling channels in order to achieve a faster through-put of test pieces.

The run-on and run-off pieces of the test piece assemblies may be 35 mm long when using the assembly shown in Figure 1. The welding parameters shall be chosen to ensure that the heat input for the fixture in use is not exceeded.

Using the welding fixture appropriate to the heat input, the test piece assembly shall be clamped in the fixture using annealed copper foil as shown.

Three or more test welds shall be made on different test piece assemblies. The deposit shall be along the centre line of the test piece assembly. The time spent in deposition shall be noted. The trailing end of the crater shall be on the run-off piece but no further than 15 mm from the central test piece. No length for the run-on portion of the weld deposit is specified, but the length shall be sufficient to achieve arc and deposit stability before reaching the central test piece. The welding fixture is used as described in 3.1.4.

The welding current, polarity, voltage, time, weld length, electrode extension, wire feed speed and gas flow and ambient temperature and humidity at the welding station shall be noted for each of the triplicate welds and recorded on the report form. After extinction of the arc and without any delay, the test piece assembly shall be released from the fixture and the test piece quenched, cleaned and stored as described in 3.1.4. For all purposes, the details listed in the report form under 3.2.3.4 shall be recorded.

1

2

3

1

2

3

Average

1

2

3

3.2.3.4 Recording of welding data and results report form

The following report sheet gives full details of all the test variables which pertain to the test results.

Report form (diffusible hydrogen, MIB, MAG, TIG or cored electrode)

Investigating laboratory: Date: Investigator's name: Type of filler material: Shielding gas: Drying treatment: Filler material designation: Gas cup i.d., mm: Diameter of filler material, mm: Shielding gas flow, l/min: Electrode polarity (d.c. +ve, d.c. -ve or a.c.): Details of tungsten electrode, if any (°C): Relative humidity …..(%) and temperature …..(°C) Make: at the welding station during welding Diameter, mm: Approximate evolution temperature: …..°C Cone angle: Hydrogen collection time: …..d; …..h Designation:

Number of test piece: Voltage, V; a.c. or d.c.: Current, A: type of meter: Welding time, s: Weld length, mm: Welding speed, mm/s: Heat input, kJ/mm: Wire feed speed, mm/s: Electrode extension, mm: Mass of deposited metal on test piece, g: Test piece to crater distance, mm:

Diffusible hydrogen (a) HD, ml/100 g of deposited metal: (b) HF, ppm of fused metal:

Other test details not included above:

3.3 Measurement of hydrogen in the test weld

3.3.1 Primary method: static method

Static method: Procedure for diffusible hydrogen collection at room temperature via an appropriate hydrogen sensor or chemical substrate

Observance of the measuring temperature is essential to exact determination of the portion of the diffusible hydrogen in ferritic weld metal. Basic specification of the degassing temperature is only possible with knowledge of the activation temperature/activation energy of the various hydrogen traps. Therefore, in order to ensure the transferability to outdated methods (mercury method), the new primary method for degassing and diffusible hydrogen collection shall generally be carried out at room temperature. For this degassing process, different designs of collecting apparatuses may be used. Degassing process is depending on used hydrogen detector.

This procedure is imperative for determining the diffusible hydrogen portions in order to reproduce the actual real diffusion behaviour in ferritic weld metal. Carrier gas hot extraction alone does not assure this transferability, since it forces diffusion processes which, due to the elevated temperature, strongly deviate in terms of time and energy from those occurring in real conditions. Elevated temperatures might activate further hydrogen sources which shall not be included, and comparatively increased hydrogen contents (outdated primary method) might be measured, respectively.

3.3.1.1 Preparation

Diffusible hydrogen collecting apparatus

3.3.1.2 Test piece cleaning

The central test piece shall be cleaned by thorough brushing to remove all slag and oxide using a steel wire brush, in good condition, with intermittent periods of cooling. The intervals spent outside the cooling bath during this operation shall not exceed 15 s.

3.3.1.3 Test piece handling

The test piece shall be removed from the storage coolant and raised to room temperature. This can be conveniently achieved by immersing the test piece in water until the ice begins to melt.

The following part of the procedure shall be carried out as quickly as possible, taking not more than approximately 2min.

NOTE Test piece handling is matching to the new apparatus!

3.3.1.4. Determination of the volume of collected hydrogen using the carrier gas hot extraction method

3.3.1.4 Analytical procedure

Maintain the test piece at 25 °C ± 5 °C, until there is no increase in hydrogen volume on successive days. "No increase" can be understood as allowing for a change, over 24 h, of no more than 1 % of the total volume collected. (depending on the hydrogen detector)

When evolution has ceased, remove the test piece from the apparatus and determine, to the nearest 10 mg, its final weight m2 in grams. Record all the relevant data.

3.3.1.5 Calculation and expression of results

3.3.1.5.1 Diffusible hydrogen in deposited metal HD

Calculate the volume HD at STP of diffusible hydrogen per 100 g of deposited metal from the following equation:

in ml per 100 g of deposited metal.

3.3.1.5.2 Diffusible hydrogen in fused weld metal HF

If the hydrogen content is required in terms of concentration in the fused metal, it is necessary to measure the cross-sectional area of the fused metal and of the deposited metal. These shall be measured on the ends of the test piece by using an enlarged tracing or photograph, or an image-analysing microscope, and then averaging the results. Diffusible hydrogen in the fused weld is calculated as shown by the following equation:

in parts per million by mass.

3.3.1.5.3 Reporting of results

All data which can be relevant to the interpretation of results shall be reported on the report form under 3.3.1.6. For the purposes of this International Standard, the average value of the hydrogen concentration of triplicate welds shall be reported to one decimal place.

The report forms given in 3.2 of this International Standard are used to report details of the welding consumable, the welding parameters and test conditions for each set of triplicate welds. The results of the hydrogen measurements shall be recorded on the same forms.

3.3.1.6 Analysis data sheet

All data required for the calculation of the diffusible hydrogen shall be recorded as follows:

Date:

Hydrogen collection temperature: ………..°C

Hydrogen collection time: ……..(14) d; ………(336) h

Room temperature (recorded during measurement) (T): °C

Number of test piece: Mass of centre test piece (m1), g: Mass of centre test piece plus weld (m2), g: Mass of deposited metal (m2 - m1), g: Average area of deposited metal, mm2: Average area of fused metal, mm2:

3.3.2 Rapid methods for the measurement of diffusible hydrogen in ferritic arc weld metal

The primary method for the measurement of diffusible hydrogen in ferritic arc weld metal is based upon collection and measurement, over appropriate hydrogen detector, of the hydrogen evolved from a standard-sized weld sample. The evolution takes place at room temperature and consequently the collection time is typically about 336 h (14 d). This time scale is acceptable in a primary method, but when results are required for purposes such as quality control, or release of a consumable for sale, then more rapid techniques of measurement are required.

In order to reduce the hydrogen evolution time, it is necessary to heat the weld sample, The heating temperature will determine the time taken for complete degassing to occur; e.g., a temperature of of 150 °C in 6 h. The higher the temperature, the quicker the degassing process. The choice of temperature is important because above about 400 °C there will be significant dissociation and release of hydrogen which, at room temperature, would remain in the molecular state and other compound forms and be permanently trapped in voids in the weld metal. At 650 °C analysis for total hydrogen, which includes residual hydrogen not measured by this reference method, can be achieved within 30 min. The condition of the sample surface has a marked effect upon the measured hydrogen volume when methods involving heating above about 500 °C are used.

It is not the objective of this subclause to describe the several alternative methods which are available for the measurement of hydrogen in metals. However, it is important to note that any alternative method incorporating the facility for measuring diffusible hydrogen in weld metal has to provide a proper correlation, in terms of accuracy and reproducibility, with room temperature diffusible hydrogen results obtained using the primary method presented in

3.3.1. When evaluating the suitability of an alternative method for the measurement of diffusible hydrogen in test welds, it is essential that the following factors be examined.

With application of the dynamic method the hydrogen measurement can be carried out at elevated temperatures (carrier gas hot extraction/melt extraction), of which the temperature level is dependent on the material or appropriate chemical substrate.

The measurement of the total hydrogen content (diffusible plus trapped) in ferritic weld metal is be carried out directly on the weld metal specimen at elevated temperatures immediately by carrier gas hot extraction/melt extraction. Hydrogen extraction is accomplished in this procedure within a much shorter period of time (rapid method). This measuring method generally yields higher hydrogen contents than the above mentioned combined primary method, since further hydrogen traps can be activated depending on the degassing temperature and on the available weld metal. Specification of an elevated degassing temperature for the diffusible hydrogen portion is therefore only possible with knowledge of the activation temperature of the hydrogen traps depending on the material to be investigated. The degassing temperature recommended in this standard for the diffusible hydrogen content is based on experience gained with ferritic weld metals and may deviate from that in an actual case. For exact determination of the degassing temperature for purely diffusible hydrogen, driving of measuring ramps at different degassing temperatures is additionally recommended in the concrete case.

Static and dynamic methods can be combined. The idea behind this combined procedure is to ensure direct comparability with results of outdated primary methods (mercury method) which will be superseded due to their health hazards.

3.3.2.1 Calibration

Calibration of analytical instruments is normally achieved by using certified reference materials to quantify instrument responses. Further, it is normal to check calibrations by performing regular analyses of reference materials or secondary standards [7].

If such reference materials are available for hydrogen, these shall be used for the calibration. In the case of hydrogen, such reference materials and secondary standards have as yet not been available because of the transitory mode of occurrence of hydrogen at, or above, room temperature.

For calibration, a method shall be used with best possible reflection of the time behaviour of hydogen diffusion from a weld metal specimen depending on the heating cycle. If a heat conductivity cell is used for the measuring principle, rapid supply of the calibration gas and associated overcontrol of the measuring signal shall be avoided. Strong extrapolation of the calibration straight line into the actual measuring range is impermissible. Special attention shall be paid to this behaviour in calibration by supply of pure hydrogen (gas calibration). It is essential during the calibration to avoid lateral losses due to different gas flow paths (e.g. by calibration before or behind the specimen If no reference material is available, the calibration must be carried out by supplying a known hydrogen volume to the carrier gas hot extraction system over the working area of interest. The analysis instruments should preferably incorporate different closed gas calibration units with different volumes. The apparatus shall be assembled and operated following the instructions of the fabricator. Depending on the analysis instrument, a sufficiently long gas flow time must be assured for stabilization. During normal instrument use, any existing calibration shall be checked by gas dosing before analysis is attempted. The instrument preparation procedure shall also be adhered to. Checks shall be made at hourly intervals during a series of analyses in order to ensure that no drift has occurred in the instrument calibration.

3.3.2.2 Linearity

The linearity of response of the instrument may be judged by using linear regression analysis to fit the calibration data and then calculating the correlation R. Values of R close to unity indicate a high degree of correlation.

The range of hydrogen contents to be measured will range from 0.05 ml to over 1 ml at STP.

Hydrogen injection shall cover this range in order to confirm linearity of response, but tests with weld specimens shall be carried out to confirm that the hydrogen evolution characteristics of the sample are followed by the instrument in a linear fashion.

3.3.2.3 Accuracy

If a reference material is available for diffusible hydrogen determination, resort shall be made to it. Accuracy can be determined by statistical comparison of preferably identical weld metal specimens. The specimens are arbitrarily divided into two groups. The first group is analysed by the alternative method whilst the second group is analysed by the primary method. The accuracy of the alternative method shall be determined at several levels of hydrogen content. These shall include the 1 ml, 5 ml, 10 ml and 15 ml per 100 g deposited weld metal hydrogen levels. A further check at the 25 ml level should be done in order to cover the analysis of non-hydrogen controlled consumables. It is recommended that nine repeat determinations be carried out using both the rapid method and the primary method described in 3.3.1. The accuracy of the rapid method is then judged by assessing the statistical significance of the difference in means of the two sets of results. If the probability of the difference not due to chance is greater than 95 %, then the difference in means is probably significant. The most common statistic to use when comparing means is the t value defined as:

The following equation may be used to calculate t

where xR, sR and nR are respectively the mean, standard deviation and number of test pieces for the rapid method and xP, sP and nP are respectively the mean, standard deviation and number of test pieces for the primary method.

The t value so obtained is applied to tables of statistics where, for the number of degrees of freedom involved (nP+ nR – 2), the probability of this value having arisen by chance will be given. If the difference in means is judged to have arisen by chance, then the two methods may be assumed to give identical results.

3.3.2.4 Reproducibility

A series of repeat welds, analysed as indicated for the test on accuracy, will also provide information on the reproducibility of the alternative method.

Reproducibility is the consistency of replicate tests and is expressed by the standard deviation s. A reproducibility index, 2s, may be defined and in statistical terms 95 % of results would lie within a band x2s where x is the mean value. A decrease in the numerical value of s implies an increase in reproducibility.

Reproducibility of a method is best determined using a planned trial in which analysts from several laboratories are involved in order to characterize the within-operator and between-operator components of the standard deviation given by the following equation:

where

sb is the between-operator standard deviation;

sw is the within-operator standard deviation.

The standard deviation of the nine results of each of the control levels mentioned under 3.3.2.3 gives values of sw for both the rapid and primary methods.

3.3.2.5 Blank

A blank shall be carried out to determine the alternative method response to a standard-sized degassed specimen. This operation is advisable on a regular basis in order to confirm proper functioning of the instrument. It should be noted that the instrument response, as shown by the increments of the readout, has an influence upon both the accuracy and the reproducibility; e.g., for a 4 g deposit weight, a readout of 0.01 ml at STP represents 0.25 ml H2 per 100 g of deposited weld, or about 0.11 ppm of fused weld.

Annex A

(informative)

Recommendations and restrictions in regard of older methods of measurement

Older methods for hydrogen degassing using mercury are no longer permissible. This applies specifically to the specimen dimensioning. O Consumables tested in accordance with this International Standard may be given a higher hydrogen classification than they would have been given under, e.g., ISO 3690:2000 or DIN 8572-1, without any change in the real product. The larger central test piece specified in this International Standard allows adequately reliable detection of still lower hydrogen levels as well as better comparison to other Standards such as ANSI-AWS A4.3-93.

Bibliography

[1]

ISO 630, Structural steels — Plates, wide flats, bars, sections and profiles.

[2]

ISO 2560, Covered electrodes for manual arc welding of mild steel and low alloy steel — Code of symbols

for identification.

[3]

DIN 8572-1, Determination of diffusible hydrogen in weld metal; manual arc welding.

[4]

Doc.IIS/IIW-452.74, Weld hydrogen levels and the definition of hydrogen controlled electrodes. Welding in

the World, 1974, Vol. 12, No.3/4, p.69.

[5]

ANSI/AWS A4.3-93 Standard Methods for Determination of diffusible Hydrogen Content of Martensitc, Bainitic and Ferritic Steel Weld Metal Produced by Arc Welding

[6]

V. van der Mee: Effect of Atmospheric Storage Conditions on Weld Metal Diffusible Hydrogen Content of Gas Shielded Cored Wires (II-1437-01)

[7]

C Ström, J. Elvander: Calibration and Verification of the Hot Extraction Method Including a Comparison with Mercury Method (II-1543-04)

[8]

F.R. Coe: Hydrogen measurements – current trends versus forgotten facts. (II-1079-86)