Authors:
Joseph Einemann, UTP Schweißmaterial GmbH & Co. KG, Germany Dr. Michael Eckhard, Krupp VDM Gmbhlor, Germany
Keywords: Welding consumables, welding consumables tests, weld filler, weld filler development, nickel alloys, special stainless steel, super austenitic stainless steel
1. Introduction
A huge number of stainless and special high-grade steels are used in the offshore- and other process-industries. A lot of products are available to weld these materials, covering a certain range of mechanical strength and of corrosion behaviours. Due to changes in specifications, such as in the customers´ guidelines, including higher demands on the welding joint, it got necessary to develop a welding filler which would cover the whole spectrum of stainless and special high-grade steels.
The Stick Metal Arc Welding, the gas-shielded arc welding TIG, MIG / MAG, or even the Submerged-Arc Welding process, are the prevailing methods in the above-mentioned fields of application. One makes use of iron-base alloys (or sometimes of nickel-base alloys, just for certain steels) as welding fillers. Weldings have been carried out with the newly developed consumable on the materials used in the mentioned fields.
Experiments have been carried out on the new Nb-free NiCr20Fe14Mo11WN welding consumable to analyse its corrosion behaviour as well as its mechanical properties. The following results show this weld filler can be brought into play to weld special and super-austenite steels. Thanks to its high yield strength, this weld filler material is usable with high-nitrogen-content alloys. The excellent results in the corrosion mediums according to the ASTM G-28A and ASTM G-48C methods, as well as in Green Death, show that the resistance to corrosion that is reached with this weld filler is comparable to other NiCrMo-alloys´one.
2. Base materials
Amongst the base materials that are used in the offshore-industry for instance, iron-based materials containing a high level of molybdenum or nitrogen are usually to be found. Table 1 shows their chemical compositions and the consequent mechanical properties. One can see in Table 1 that these materials have high strength properties, which again the weld joint should at least reach, or rather go beyond.
3. Welding filler materials
As said at the beginning, the materials quoted in Table 1 can be welded by different filler metals. Table 2 shows some typical analyses of these welds, usually used to join the materials of Table 1.
Duplex and Super Duplex alloys are normally welded with similar fillers. For high-molybdenum- and nitrogen-alloyed steels (super austenite), the NiCr20Mo9Nb or NiCr2Mo16 are used.
The setting of the NiCr20Fe14Mo11WN has been conducted to make it possible to use only one weld filler metal for all the base materials and their combinations to come in the mentioned fields. This is going to have some positive effects for the workmen as far as quality assurance is concerned, in so far as the risks of building-site and fabrication line mistakes are considerably reduced.
4. Tests
Some experimental weldings were conducted with the base materials quoted in Table 1 and the weld filler NiCr20Fe14Mo11WN, with stick electrodes and the shielded-gas wire. The production of samples occurred in the PA, PF, and PD welding positions for the stick electrode, and in the PA position for the gas-shielded welding.
The plates were machined with a V-groove angle of 70° for thicknesses of 12 mm to 16 mm. The strength properties were checked by taking samples from the pure weld metal and from the joint and by testing them at room temperature as well as at higher temperatures of 450°C and 550°C. The respective ground materials were invariably tested together, to make sure the testing conditions were the same for all samples and the results would be comparable.
Corrosion and mechanical test samples were taken afterwards from these test sheets to be checked in chemical solutions, according to the following test specifications:
ASTM G-28, Method A:
50 % H2SO4, 41.7 g/lFe2
(SO4)3X9H2O, water
ASTM G-48, Method C:
6 % FeCl3, water
„Green Death“:
6.4 % H2SO4, 3.1 % HCl, 1 %
CuCl2, 0.6 % FeCl3, water
The solution according to ASTM G28A was boiling, and the starting temperature of ASTM G-48C and Green Death amounted to 25°C. After the first period of experiment, temperature got increased by 2.5°C. This process was repeated until the specimens showed marks of pitting corrosion. The experiment lasted 5 days within the scope of the ASTM G-28 method A, and 1 day per temperature period as far as ASTM G-48C and Green Death are concerned.
All the test pieces had been etched before testing until the welding traces disappeared, i.e. in other words immersed at room temperature in a 40-min bath composed of 5 % HF + 20 % HNO3
+ H2O.
After the experiments, the loss of mass was determined and converted into mm/a on a loss rate. All pieces were the object of both a visual and a microscopical examination, in order to try and check all the zones submitted to local corrosion attacks, and, if necessary, to describe them in greater detail.
When considering the losses of mass, it is important to compare these values with the base material ones, each time greater as the weld only represents around 1/10 of the total surface cut. It is forbidden to convert the loss of mass that occurs through local corrosion, as during the Green Death tests in particular, into a loss rate, in so far as in this case the loss of mass is obviously to be attributed only to local attacks in the weld metal, fusion line and heat affected zone. Thus, if this particular loss of mass was attributed to these smaller areas of weld filler, the corrosion attack would be misassessed, as too much prominent.
5. Results and interpretation
5.1 Corrosion examination
5.1.1 Duplex X2CrNiMoN22 5 3, material No. 1.4462
In the penetrant solution G-28A, a better quality was achieved with the welded joint than with the base material. The improvement of the weight loss values comes from the higher welding filler alloy.
The tests ASTM G-48C and Green Death cause unhomogeneities such as segregation through pitting corrosion in the heat affected zone, or rather the fusion line. The specimens were resisting a temperature of 35°C to 40°C. This value corresponds to the properties of the base material.
The respective data are quoted in Fig 1.
5.1.2 Super Duplex X2CrNiMoCuWN25 7 4, material No. 1.4501
With ASTM G-28A, the weight loss rates with the base material were lower than those obtained with the combination. The more prominent loss of mass is associated with the welding, or rather with the heat input.
In ASTM G-48C and Green Death the same values were achieved on both the untreated base material and the welded joint.
See the values in Fig. 2.
5.1.3 X1NiCrMoCuN25 20 6, material No. 1.4529
One can compare The weight loss values of the untreated base material and of the welded joint in ASTM G-28A. Pitting corrosion appeared on the fusion line of the weld specimens in ASTM G-48C and Green Death. The pitting temperatures are comparable too for welded and untreated test pieces.
Results gathered in Fig.3.
5.1.4 X3CrNiMnMoNbN23 17 5 3, material No. 1.4565
The values obtained with the 3 corrosive solutions are comparable, for welded and untreated test pieces.
See all results in Fig.4.
5.2 Mechanical properties
Figures 5 to 8 gather all the results obtained. It is obvious that the yield strength reached with the NiCr20Fe14Mo11WN filler metal is high (RP 0,2 ca. 500 MPa). The nitrogen-alloyed 1.4462, 1.4524 and 1.4529 base materials do not reach this strength. The super duplex alloy goes approximately 150 MPa beyond the results obtained with the electrode welded joint or even with the MIG/MAG process.
However, the TIG-process results show only a difference of approximately 80 MPa. The location of failure was in the base material, and for the super duplex, in the joint. The weld metal may have a cast structure thanks to the cooling conditions, elongations of 35 to 45% happen all the same with this alloy. Those important base material elongations (35 to 60%) have to be associated with the solution tests in heat-treated conditions. These results can only be achieved with a stress relieved structure and if the alloy is in a homogenic environment. Elongation rates inevitably appear on the base material and the weld filler metal, and the base material consequently has more influence on the established results, due to the specimen geometry. The specific resulting rates then approximately represent 15 to 40 %, depending on the base material type.
The notch ductility, tested on Charpy-V-notch specimens at + 20°c and -196°C, reaches as well 60 to 160 J, depending on the welding process employed. To establish a further qualification of the welded combinations, bend and side bend test pieces on the root and the surface layer have been examined. The obtained bend angle reached approximately 180° for all the welded joints and test pieces, where a bend mandrel of 3-test-piece diameter had been used.
5.3 Hot-cracking resistance
The hot-cracking resistance has been tested by the federal office for material research and test (Bundesanstalt für Materialforschung und Prüfung) in Berlin on MVT-test pieces. In comparison to other nickel-base welding filler materials, the newly developed weld filler NiCr20Fe14Mo11WN showed good results. This is true for the heat inputs of 7,5 + 14,5 kJ / cm normally used during this test.
6. Summary
What distinguishes the newly developed weld filler NiCr20Fe14Mo11WN is its high yield strength, combined at the same time with extremely good corrosion behaviours. The above analyses show that this weld filler material makes the welding of such metals as high-nitrogen and stainless steels easy. The corrosion results in ASTM G-28A, ASTM G-48C and Green Death show that the base material has a considerable influence on resistance. However, the transition from weld metal to base material and the heat affected zone keep being the shortcomings in these steels (duplex, super duplex and special high alloyed steels).
The mechanical properties of the weld and of the joint with 3-material combination (1.4529, 1.4565 and 1.4462) must be considered as comparable. What concerns the rates achieved with super duplex, a yield strength reduction of merely 80 MPa has to be taken into account. The above results verify the fact that this material can be applied to a wide range of uses. We are convinced that the use of weld filller materials in further fields is possible, where other NiCrMo alloys do still prevail today, for instance.
Acknowledgement:
Thanks to the R & D department employees in UTP Bad Krozingen, UTP Hamm and Krupp VDM, for their support and encouragement.
