Supermartensitic stainless steels (SMSS), including both lean grades (without molybdenum) and high grades (with 2% molybdenum), have shown sensitivity to intergranular corrosion or stress corrosion cracking (IGSCC) in weld HAZs during laboratory testing in chloride solutions acidified with CO2 at above about 100°C(1-9). Also, lean SMSS grades have cracked in service when exposed to hot acidic brines(4,6). This paper describes a Joint Industry Project designed to test existing understanding of the mechanism of sensitisation to IGSCC of welded SMSS. The project also sought to determine whether IGSCC could be reliably avoided via control of the welding process or by brief PWHT. Existing data were reviewed and two series of welding trials and corrosion tests were undertaken. The review and first series of tests allowed a hypothesis of the mechanism of sensitisation in SMSS to be developed, which was then tested via the second series of tests. Several SMSS grades were examined. Welds were made with a range of welding thermal cycles, by varying the heat input and interpass temperature and by use of active water cooling. Some welds were given PWHT at 650°C for 5 minutes. Stressed bend specimens from the welds were tested in 25% NaCl solution acidified with 10bar CO2 at 110°C. Some tests included a small addition of HCl, to acidify the solution to pH 3.3 prior to introduction of CO2. From the results of the tests and measurements of the weld thermal cycles, it was concluded that a complete understanding of the mechanism of IGSCC in supermartensitic steels does not yet exist, although the main contributing factors are known. Consequently, where the service environment is sufficiently aggressive, the risk of IGSCC cannot be entirely eliminated by control of the welding process alone, although brief PWHT offers a practical means of avoiding IGSCC.
A review of published information indicated that many but not all girth welds in SMSS, in the as-welded condition, are susceptible to stress corrosion cracking in hot acidic brines, by an intergranular mechanism following prior austenite grain boundaries(1-6). The crack appearance suggests that sensitisation occurs at specific HAZ locations rather than throughout the HAZ. Specimens with a machined root or ones given brief PWHT at 650°C for 5 minutes were not found to be susceptible to cracking(7). No clear links to HAZ hardness, HAZ microstructure or composition were found for the alloys tested and it was not clear how widely applicable the beneficial effect of PWHT was.
The most commonly observed macroscopic cracking mechanism follows prior austenite boundaries, therefore a hypothesis was developed which proposed that carbide precipitation on these boundaries provides the main mechanism for sensitisation. At least two thermal cycles are required, with the first thermal cycle forming fresh martensite and the second and subsequent cycles forming the damaging carbides.
Therefore, a series of welds was made with different heat inputs, and corrosion tests performed, to see if a distinction could be made between welds that were susceptible to IGSCC and those which were not, on the basis of welding procedure.
