The influence of welding parameters on the sensitisation behaviour of 3CR12

by Greeff, Mary Louise.

The sensitisation of a 12% chromium ferritic stainless steel, conforming to EN 1.4003 and available commercially in South Africa under the trade name of 3CR12, was investigated during the course of this project. 3CR12 was designed to pass through the (?+?) phase field on cooling, with the austenite transforming to martensite on subsequent cooling to room temperature. The aim of this investigation was to verify that 3CR12 can sensitise during continuous cooling after welding, provided that low heat input levels are used. Two grades of 3CR12 with slightly different chemistries, designated 41220 (A) and 41311 (B), were evaluated. Grade 41220 has a higher austenite potential than grade 41311. 3CR12 plate was joined autogenously to AISI 316L by means of a series of square butt welds. Gas tungsten arc welding with argon shielding was used, and the heat input was varied from approximately 30 J/mm to 450 J/mm, in conjunction with welding speeds ranging from 2.36 mm/s to 33.3 mm/s. Rosenthal’s heat flow equations were used to calculate the cooling rate from 1500°C to 800°C for each experimental weld, and to illustrate the influence of the effective heat input and welding speed on the martensite content of the high temperature heat-affected zone. An increase in welding speed reduces the heat input and increases the cooling rate after welding. At lower heat input levels (less than approximately 100 J/mm), austenite nucleation was found to be suppressed by the rapid cooling rates, and a continuous network of ferrite-ferrite grain boundaries formed in the high temperature heat-affected zone. Higher heat inputs resulted in slower cooling with more martensite in the high temperature heat-affected zone after cooling. At heat input levels above approximately 250 J/mm, enough martensite formed during cooling to eliminate a continuous network of ferrite-ferrite grain boundaries in the high temperature heat-affected zone. Sensitisation was evaluated using an electrolytic oxalic acid etch (ASTM 763-99, Practice W), and a potentiostatic etch in 0.5M H2SO4. During the potentiostatic etch test, the potential was maintained at 0 VSCE to reveal the presence of any chromium depleted zones. Both grades of 3CR12 were found to be sensitised when a continuous network of ferrite-ferrite grain boundaries was present in the high temperature heat-affected zone (i.e. after welding at low heat input levels). When the heat input during welding was high enough to ensure the presence of martensite on the majority of the heat-affected zone grain boundaries, thereby effectively eliminating continuous ferrite-ferrite grain boundary networks, the welds were not in the sensitised condition. The austenite that forms during cooling acts as a carbon sink, absorbing any excess carbon. This prevents supersaturation of the ferrite and subsequent carbide precipitation that can lead to chromium depletion and sensitisation. Due to its higher austenite potential, grade 41311 can be welded at lower heat input levels and with faster cooling rates than grade 41220 without inducing continuous carbide precipitation and sensitisation. In order to prevent sensitisation, a fusion-line cooling rate of 80°C/s should not be exceeded in 3 mm 3CR12 plate during welding.