Improvement of HAZ Toughness for High Heat Input Welding by Using Boron Diffusion from Weld Metal
Yasushi KITANI*, Rinsei IKEDA*, Koichi YASUDA*, Kenji OI** and Katsuyuki ICHIMIYA***
* Steel Resea rch Laboratory, JFE Stee l Corporation 1 Kawasaki-c ho, Chuo-ku, Chiba 260-0835, Japan ** Steel Rese arch Laboratory, JFE Stee l Corporation 1 Kokan-cho, Fukuyama, 721-8510, Japan *** Steel Research Laboratory, JFE Stee l Corporation 1 Kawasaki-dori, Mizushima, Kurashiki 712-8511, Japan
Abstract :
In h igh heat input welding, such as 1-pass e lectro-gas arc welding (EGW) of thick steel pla tes, HAZ toughness is deteriorated because HAZ microstructure coarsens significantly. TiN precipitates dispersed in steel plates are effective to suppress coarsening HAZ microstructure by pinning effect. However TiN precipitates resolve in high temperature region nea r fusion line, whe re deterioration o f HAZ toughness is unavoidable near fusion line.
As a solution of this problem, authors paid attention to boron in weld metal. Boron is k nown to be effective to prevent coarse ferrite formation at prior austenite grain boundaries and to diffuse rapidly in steel. In this research, boron diffusion from weld metal to HAZ during EGW process was confirmed by SIMS analysis. It was certified that diffused boron in HAZ improved HAZ toughness because boron suppressed microstructure coarsening and fixed free nitrogen originated by TiN resolution as BN near fusion line. These research results proposed a new HAZ structure control technology using the matching high-B bearing weld metal with TiNtreated steel plates.
1. Introduction
Recent ye ars a remarkable trend toward large-scale welded structures of ships and buildings has been seen. For example, in the field of shipbuilding, adoption of large-sc ale conta iner ships has progressed ra pidly against the background of a rising volume of container cargo in demand for the increased long-dista nce distribution, and large vessels of the 8000 TEU (twenty-foot equivalent unit) clas s have been constructe d recently. In the constr uction industry, high-rise and large-spa n construction becomes c ommon in the modern buildings seen in urban redevelopment plans, requiring the large-section s teel frame structures.
Welded structures of these type s are fabricated using higher strength and heavier thickness s teel plates than thos e in the past. At the sa me time , r equirement for the toughnes s of the w elde d joints has become increasingly strict to insure safety and to prevent environmental damage due to accidents. To keep productivity in welding heavy steel plates, there is a strong need for high heat input welding processes such as ele ctrogas arc welding (EGW), e lectroslag we lding (ESW) and multi-electrode submerge d arc welding (SAW).
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In high heat input welding of heavy steel plates, HAZ microstructure coarsens remarkably because the heat affec ted zone (HAZ) is held at high tempera ture f or a long period. In particular, the bond toughnes s is mar kedly reduce d. A technology that utilize s the pinning effect of TiN partic les finely dispersed in the base metal steel has been developed to restrain coarsening of the HAZ microstructure and improve bond toughness. However coarsening HAZ microstructure cannot be prevented completely because of TiN dissolution near the fusion line.
To solve this problem, the authors focused on the e lement boron (B), which is known to be effe ctive in improving bond toughne ss, and investiga ted a new technology for s ecuring high HAZ toughness in high heat input welding ba sed on diff usion of B from the weld metal (WM). The results are reported in the following.
2. Concept of Improving HAZ Toughness in High Heat Input Welding
As an example of high heat input welding, electroga s arc welding(EGW), as shown in Fig. 1, is used in block welding of container ship shear strakes and hatch coamings. Typically, 1-pass vertical-up position welding is performe d on extra-he avy ste el plates with thicknesses of 60mm or more. In 1-pass welding with 65 mm thic k plate, show n in Fig. 2, the w elding heat input ros e to 50-60 kJ/mm a nd the w eld w as he ld at high temperature for a long period. In this case, the cooling time (800 -500ºC) in the HAZ near the fusion line was on the orde r of 400 s.
In suc h a high he at input we lding, aus tenite grain gr owth in the HA Z be come s remarkable a nd the coarse-grain austenite region expands as shown in Fig. 3. Because the coarse-grain microstructure consisting of ferrite side plate s at prior austenite grain boundaries and intra-gra nular upper bainite has low toughne ss, HAZ toughness is greatly reduced.
As a solution to the problem of reduced HAZ toughness, methods of suppres sing HAZ coarsening have studied from an early date. One of the most effective me thods is utilizing the pinning effect of fine grains of nitrides, c arbides, etc. dispersed in the base steel plates. Fig. 4 shows the ef fect of TiN addition to the base plate on suppressing HAZ coarsening. In a high heat input EGW joint of 65 mm thick plates welded with a heat input of 50 kJ/mm, the coarse-grain region in the HAZ can be greatly reduced by proper TiN addition, preventing deterioration of HAZ toughness.
However, there are limits to HA Z toughness improvement by this method of TiN addition. In particular, coarsening microstructure cannot be adequately prevented in the bond area, which comprises an area of several
| Fig. 1 | Sche matic illustra tion of simplif ied | Fig. 2 Exa mple of cross section of EGW weld. |
| electrogas arc welding. | (Plate thickness : 65mm) | |
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Table 1 Chemical composition of steel used. (Plate thickness : 65mm)
(mass%)
*
C SiMnP S B
Note
C eq
0.08 0.19 1.5 0.006 0.001 <0.0001
0.35
Ti treated
*
Ceq = C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14 (WES)
Table 2 Welding conditions of EGW.
Plate thickness (mm)
Groove angle (deg.) Root gap (mm) Welding current (A) Arc voltage (V) 
Heat input (kJ/mm) 50～60
Fig. 5 Weld groove of EGW.
65 20 8 420 42
analysis by secondary ion mass spectrometry (SIMS) and -ray track etching observation by irradiation. In the analysis of precipitated B in the WM, the thermal cycle in Fig. 6 was applied to WM sa mples taken from the pr epar edjoints, and the differe nce between the total B content and precipita ted B(insoluble B) after the ther mal cycle was obtained. This differenc e was considere d to be the amount of free B that is a vailable for diffusion under he ating at 1,350ºC. In B a nalysis by SIMS, as show n in Fig. 7, 100 m squa re sections of the HAZ were scanned with an ion beam at intervals of 200 m from around the fusion line. B/Fe secondary ion intensity ratio was obtained by integrating the ratios of the detected intensity of B to Fe in each area. This B/Fe ra tio was rega rded as the relative B concentration a nd used to indicate the B distribution. In -ray tra ck etc hing, a sample w ith a resin film attac hed to its surfa ce wa s exposed to radiation, and the condition of B existence in the HAZ around the fusion line was estimated from the discharge tracks of isotope s of B with an atomic weight of 10, that could be obs erve d as images on the film. In all cases, the samples used in these analyses were 1/4t (plate thickness) position weld samples ta ken from the prepared we lded joints.
Fig. 6 Thermal cycle for insoluble boron Fig. 7 Analysis method for boron distribution around analys is in weld metal. fusion line using SIMS.
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The HA Z toughness in actual joints was eva lua ted by the Charpy 
impac t test (te st pie ce shape: JIS Z 2202 No. 4, test temperature : -20 ºC)
with notches introduced in the center
of the we ld meta l a nd on the fusion line at the 1/4t position, as illustra ted in Fig. 8. Fig. 8 Notch location of Charpy impact test of EGW we lded joint.
4. Results and Discussion
Fig. 9 shows the contents of insoluble (precipitated) and soluble (solid solution) B obtained with the WM thermal cycle samples. The WM had a totalB content of 37 ppm. In contrast, when the WM is heated to 1,350ºC, it can be e stimated that 24 ppm, or approximately 70% of total B exists in the form of free B ava ilable f or diffusion. In high heat input EGW, the
(Total B in
hea t input exc ee ds 50 kJ/mm a nd the WM is held a t a
weld metal
tempera ture over 1,350ºC after solidification for quite a long
= 37ppm)
time. It is therefore thought that a B equal to or grea ter than the soluble B obtained in this analysis is available for diffusion. Fig. 9 Soluble and insoluble boron amount
Fig. 10 shows the B concentration distribution in the in weld metal at temperature WM and HAZ around the fusion line by SIMS analysis. The B concentration in the HAZ increases approaching the fusion line, confirming the fact that B has diffused from the WM to the base metal, which did not contain B. Furthermore, a higher concentration of B was found near the fusion line ne xt to WM with high B c onte nt (B content of WM = 33 ppm), show ing a diffe renc e in B diffusion depending on the difference in the B content of the WM. Here, if the B concentration at the respective positions is thought to be proportional to the B intensity ratio detected by SIMS analysis, approximately 1/10 of
-500 0 500 1000 Distance from fusion line (μm)
Fig. 10 Boron distribution around fusion line. (analyzed by SIMS)
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B content in WM, or several ppm of B, exists even at a position 300-400 m from the fusion line. It is sufficiently possible that defused B from WM influences the transformation behavior of the HAZ during the cooling process in the welding thermal cycle.
Fig. 11 shows the results of observa tion of B images by -ray tra ck etching of a weld with ba se metal with B content of less than 1 ppm and WM with B content of 24 ppm. Because the B content in WM is high, a high concentration ca n also be obs erved in the images, wherea s in the base metal, which did not conta in B, a very small amount of B precipitation at prior austenite grain boundaries can be observed. In the HAZ near the fusion line, granular B can be observed in the range of 400-500 m from the fusion line, and it can be assumed that diffuse d B from the WM exists as a precipitate in the form of BN.
Based on the analysis of precipitated (insoluble) B in the WM, SIMS analysis of B distribution and ray track etching observation, it can be concluded that B diffuses from WM with high B content to the HAZ in the high heat input EGW, and furthermore, substantial B diffusion exists over a distance of approximately 300400 m.
Fig. 12 and 13 respectively show the Charpy impact test results and the microstructure around the fusion line in two kinds of EGW joints with low B content (11 ppm) of WM and high B content (40 ppm) of WM. The Charpy impac t test results in Fig. 12 reveal no difference in WM toughness in the twojoints, but thejoint with high-B WM cons istently displays higher bond toughnes s values. In the Charpy te st of the bond w ith low-B WM, the fracture starting point of the test piece with low absorbed e nergy (vE-20ºC<50J) was locate d in the coarse-grain part of the HAZ, as shown in the microstructure photograph in Fig. 12. It is therefore thought that the toughness of this part was improved by matching with the high-B WM. In the comparison of microstructures in Fig. 13, in the s ample with the high-B WM, forma tion of coarse ferrite at prior austenite grain bounda ries was suppressed in the coars e-grain region of the HAZ near the fusion line. In other words, a microstructure improvement effect was confirmed in the coarse-grained part, which serves as the fracture starting point in the Charpy impact test.
As described above, a phenomenon in which B diffuses from the WM during high he at input EGW was confirmed. And test results showed that the microstructure and the toughness of the coarse-grain HAZ near the fusion line were improved by B diffusion from WM. The mec ha nism of the improve me nt of the H AZ microstructure and toughness is explaine d as follows.
Fig. 11 Obs ervation of boron status in HAZ. ( α -ray track etching image)
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Fusion line Fusion line
(a) B c ontent in WM : 11ppm (b) B c ontent in WM : 40ppm
Fig. 14 shows a schema tic illustration of the behavior of B and pr ecipitates in the HAZ near the fusion line with high-B be aring WM. Near the fusion line in high heat input welds, the TiN disperse d in the base steel resolves in the region which is heated to high temperature exceeding 1,400ºC. In the area where TiN resolves, austenite grains coarsen and the free N increases accompanying TiN resolution. In such a coa rse-grain HAZ, coarse ferrite or ferrite side plate structure is formed at the austenite grain bounda ries and increased free N exists in the matrix. Therefore HAZ toughness near the fusion line is de teriora ted remarka bly. Howeve r,
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diffusion of a sufficient amount of B from the high-B bearing WM to the HAZ during welding has the following effects: (1) Formation of coarse fer rite or ferrite-side-plates is suppres sed by segregation of B at the pr ior austenite grain boundaries, and (2) free N is fixed by formation of B N. As a result, the microstructure and the toughness in the coarse-grain HAZ a re improved.
In other words, by matching a high-B bearing WM and utilizing B diffusion from the weld, it is possible to prevent toughness deterioration in the coarse-grain HA Z near the fusion line, which had been a problem with TiN-tre ated ste els. This concept differs from the c onve ntional HAZ toughnes s improvement te chnique of optimizing the base plate chemic al composition, and demons tra tes the possibility of a funda mentally new HAZ structure control technology based on the matching of the WM and the base plate.
4. Conclusion
The phenomenon of B diffusion from the WM to the HAZ in high heat input w elding was verified, and the mechanism of improvement in HAZ toughness by B diffusion from the WM was investigated. The following conclusions w ere obta ined.
- (1)
- Diffusion of B from the WM to the HAZ was confirmed in welds in high heat input EGW joints. When the HAZ is contiguous to high-B WM, B content of several ppm or more exists in the range of 400-500 m from the fusion line and has a beneficia l effect on the HAZ mic rostructure.
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