Stress Corrosion Cracking of Welded Joints in High Strength Steels Variables affecting stress corrosion cracking are studied and conditions recommended for welding and postweld heat treat to obtain maximum resistance to cracking BY T. G. GOOCH ABSTRACT. High strength steels may linear elastic fracture mechanics prin­ detrimental effect on weld metal SCC suffer a form of stress corrosion ciples using precracked specimens. resistance, although segregation may cracking (SCC) due to hydrogen em­ Testing was carried out in 3% sodium be particularly significant in precipita­ brittlement, the hydrogen being liber­ chloride solution as representative tion hardening systems. SCC failure ated by a cathodic corrosion reaction. of the media causing SCC of high may take place intergranularly, by Most service media will be expected strength steels. Welds were prepared cleavage, or by microvoid coales­ to liberate hydrogen, and the problem in the experimental alloys and the cence, intergranular failure being affords a considerable drawback to pre-existing crack located in various largely associated with the presence the widespread use of high strength regions of the joint, while samples of twinned martensite and high sus­ steels. For a number of reasons, fail­ were also prepared using The Weld­ ceptibility. The results suggest that ure may be particularly likely when ing Institute weld thermal simulator highest SCC resistance will be ob­ welding is used for fabrication. Un­ to reproduce specific heat-affected tained from low carbon, low alloy less the structure is efficiently stress zone (HAZ) microstructures. Suscep­ systems, since these are unlikely to relieved, tensile stresses of yield or tibility was defined in terms of the suffer the development of deep corro­ proof stress magnitude can remain in critical threshold stress intensity to sion pits which may act to initiate the vicinity of the weld, while even cause cracking, the results obtained SCC in service, and will have high with stress relief, a weld will consti­ being related to material composition resistance to SCC by a cleavage tute a region of stress concentration. and microstructure with reference to mechanism. During the welding cycle, metallur­ different heat affected zone and weld Attention has been further paid to gical changes may take place causing metal areas. Fractographic examina­ the practical risk of SCC initiation in local microstructures that are particu­ tion was carried out and susceptibility service, consideration being given to larly sensitive to SCC. The present related to failure mechanism. the effects of environment on SCC investigation was initiated to clarify From the results obtained it has initiation and propagation. A prelim­ the situation, three particular objec­ been possible to derive a general inary correlation has been obtained tives being identified, namely: understanding of the variables affect­ between fracture mechanics data and 1. To define the relative SCC be­ ing weld SCC performance. Suscep­ the SCC behavior of uncracked, un­ havior of a range of high strength tibility is primarily dependent upon dressed welds. steels. microstructure, and, for a given mate­ The information reported thus con­ 2. To assess the effects of welding rial, recommendations may be made stitutes a rational basis for under­ and postweld heat treatment on regarding welding conditions and standing the SCC behavior of welded SCC susceptibility. postweld heat treatment to obtain joints. Although direct testing is still 3. To evaluate the general prac­ maximum SCC resistance. The necessary to establish the behavior of tical implications of the data ob­ presence of twinned martensite in a given joint in a particular environ­ tained. particular should be avoided, since ment, it is possible to maximize SCC A range ot steels was studied in­ this ohase has a highly deleterious resistance in advance by attention to cluding both conventional medium effect on SCC resistance. Provided relevant factors. carbon, low alloy materials and low postweld heat treatment similar to Introduction carbon, precipitation hardening that specified for base metal is ap­ grades. SCC testing was based on plied, HAZs and matching composi­ While it is widely realized that fer­ tion weld metals will generally show ritic steels may be subject to stress T. G. GOOCH is Group Leader — SCC resistance similar to the base corrosion cracking (SCC) in certain Austenitic Group, The Welding institute, metal; without such heat treatment, Abington, Cambridge, Great Britain. Paper specific environments such as caustic increased susceptibility must be an­ conditions (Ref. 1), the environments was presented at the 55th AWS Annual ticipated. The presence of segregation Meeting held in Houston during May 6-10, causing trouble have in the past been and inclusions generally has little 1974. relatively few and the problem has WELDING RESEARCH S U PP LE M E NT I 287-s not been widespread. However, havior of welds in high strength practical application (Ref. 7). Further, during evaluation of high strength steels. Although a number of evalua­ the crack can be sited in any region of alloys (yield stress > 1000 tion studies have been carried out a weld, and the risk of failure of in­ N/mm2(150 ksi)) for aerospace and relevant to specific alloys welded dividual weld regions assessed large­ other applications, it became appar­ using particular techniques, the rela­ ly independently of surrounding mate­ ent that these materials could suffer a tive importance of the various factors rial. Concurrently with these studies, more general form of SCC in a variety determining weld performance has metallographic and fractographic of media at around room temper­ not been defined. In consequence, it examination was carried out to iden­ ature, cracking arising in environ­ has been difficult to make general tify the factors determining SCC ments that had proved entirely recommendations for the avoidance behavior. innocuous in the past with lower of SCC in service. This lack of knowl­ Second, SCC testing of plain un- strength alloys (Ref. 1). The reason edge constitutes a potential drawback cracked welds was undertaken, and for this susceptibility has been to the more widespread use of the results correlated with the frac­ studied, and two hypotheses pro­ welded medium and high strength ture mechanics data. Compositional pounded, namely that failure is due steels, especially since a very wide and microstructural gradients across either to the presence of a readily range of environments has been an unmachined, uncracked weld may corrodible "active path" within the reported to cause cracking (Ref. 5). modify SCC initiation and propaga­ material, or to a form of hydrogen em­ The present investigation was under­ tion as compared to a precracked brittlement, with the hydrogen being taken to clarify the situation, three specimen where the development of liberated by a cathodic corrosion reac­ particular objectives being identified: SCC is largely controlled by the pre­ tion (Ref. 2). While the former mech­ existing crack and associated plastic anism may be operative in some in­ 1. Evaluation of the relative SCC zone. Further, there is little informa­ tion on the extent to which geomet­ stances, research has indicated that behavior of a range of high rical stress and strain concentrates at in general failure takes place as a strength steels. weld toes affect the corrosion proc­ result of hydrogen embrittlement (Ref. 2. Definition of the effects of esses causing SCC initiation. Thus, it 3). From investigations into the prob­ welding and postweld heat treat­ was considered necessary to estab­ lem of hydrogen embrittlement of ment on SCC susceptibility. lish that relative data obtained from steels, the risk of SCC can be expect­ 3. Assessment of the general practical implications of the data precracked samples were directly ed to increase with increasing mate­ applicable to practice. rial hardness, and the phenomenon obtained. assumes particular significance in Experimental Approach Precracked Sample Studies the case of high strength alloys. Sample Preparation Stress corrosion failure at welded Five high strength steels were joints in high strength steels may be selected as a basis for study, includ­ The base metals were heat treated especially likely for a number of ing both medium carbon, low alloy as in Table 2. Single edge notched reasons (Ref. 4). Unless the fabrica­ material and low carbon, precipita­ and fatigue cracked (SEN) samples tion is efficiently stress relieved after tion hardening grades. The alloys (Ref. 7) were machined as in Fig. 1, welding, residual welding stresses chosen were FV520S, NCMV, with adequate dimensions in all will remain. In the weld area, these RS140, 18% nickel maraging, and a cases to give plane strain condi­ stresses will be tensile, and as they copper-silicon steel Table 1, first five tions during SCC testing, although will normally be of yield stress magni­ items), with the materials supplied as this was not always the case with tude, they can greatly increase the 25 mm (1 in.) and 15 mm (0.6 in.) toughness tests in air (Ref. 8). risk of cracking. Even with stress thick plate (Ref. 6). Other, lower Welds were made in each steel to relief treatment, local stress concen­ strength alloys were used in the cover a range of processes of indus­ trations at welds will usually remain. course of the investigation to clarify trial application as indicated in Table Depending on detail design, welded certain aspects of