Pitting and stress corrosion cracking behavior in welded austenitic stainless steel

Role of δ-ferrite in stress corrosion cracking retardation near fusion boundary of 316NG welds

The influence of the weld metal chemistry on the susceptibility of AISI 444 ferritic stainless steel (FSS) weldment to stress corrosion cracking (SCC) in hot chloride was investigated by constant load tests and metallographic examination. Two types of filler metal of austenitic stainless steel (E316L and E309L) were used in order to produce fusion zones of different chemical compositions. The SCC test results showed that the interface between the fusion zone (FZ) and the heat affected zone (HAZ) was the most susceptible region to SCC. Results also showed that the AISI 444 stainless steel weldment with E309L weld metal presented the best SSC resistance. Microstructural examinations indicated that the cracks initiated in the weld metal and propagated to the HAZ of the AISI 444 FSS, where the fracture occurred and it was observed a considerable amount of precipitates. Additionally, the higher SCC resistance of the AISI 444 FSS weldment with E309L weld metal may be attributed to the presence of a discontinuous delta-ferrite network in its microstructure, which acted as a barrier to cracks propagation from the fusion zone to the HAZ/fusion zone interface of AISI 444 FSS. Fractrography analyses showed that the transgranular quasi-cleavage fracture mode was predominant in the AISI 444 weldment with E316L weld metal and the mixed fracture mode was the predominant in the AISI 444 weldment with E309L weld metal.

Effect of weld metal chemistry on stress corrosion cracking behavior of AISI 444 ferritic stainless steel weldments in boiling chloride solution

: A variation of FSW, friction stir processing (FSP), has been used to modify selected regions of materials to enhance specific properties while eliminating fusion welding defects such as porosity, cracking, and the cast microstructure. The combination of fusion welding defects and high tensile residual stresses caused by the solidification of the molten weld pool adversely affect the post weld service integrity. FSP has been demonstrated to eliminate many of these problems while at the same time improving the resulting properties. FSP has been utilized to locally process regions of arc weldments in 304L stainless steel to improve the service integrity. The cast microstructure and coarse delta-ferrite is replaced with a fine-grained wrought microstructure. This paper presents a preliminary processing window for FSP of 304L stainless steel, the resulting microstructure associated with this process window, along with considerations given to mechanical properties and corrosion.

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Effects of Friction Stir Processing on the Microstructure and Mechanical Properties of Fusion Welded 304L Stainless Steel

Stainless steels have good corrosion resistance in many environments but welding or aging can decrease their resistance. This work focused on the effect of aging time and ultrasonic nano-crystal surface modification on the passivation behavior of 316L stainless steel. In the case of slightly sensitized 316L stainless steel, increasing the aging time drastically decreased the pitting potential, increased the passive current density, and decreased the resistance of the passive film, even though aging did not form chromium carbide and a chromium depletion zone. This behavior is due to the micro-galvanic corrosion between the matrix and carbon segregated area, and this shows the importance of carbon segregation in grain boundaries to the pitting corrosion resistance of stainless steel, in addition to the formation of the chromium depletion zone. UNSM (Ultrasonic Nano Crystal Surface Modification)-treatment to the slightly sensitized 316L stainless steel increased the pitting potential, decreased the passive current density, and increased the resistance of the passive film. However, in the case of heavily sensitized 316L stainless steel, UNSM-treatment decreased the pitting potential, increased the passive current density, and decreased the resistance of the passive film. This behavior is due to the dual effects of the UNSM-treatment. That is, the UNSM-treatment reduced the carbon segregation, regardless of whether the stainless steel 316L was slightly or heavily sensitized. However, since this treatment made mechanical flaws in the outer surface in the case of the heavily sensitized stainless steel, UNSM-treatment may eliminate chromium carbide, and this flaw can be a pitting initiation site, and therefore decrease the pitting corrosion resistance.

Effect of Ultrasonic Nano-Crystal Surface Modification (UNSM) on the Passivation Behavior of Aged 316L Stainless Steel

Austenitic stainless steel welds with different ferrite contents were produced using the submerged arc welding (SAW) strip-cladding process. The clads were removed and remelted using the gas tungsten arc welding (GTAW) process. They were post-weld heat treated at 600, 800 and 1000 °C for 1, 10 and 100 h. Stress corrosion cracking (SCC) tests were conducted on notched tensile samples in a 5N H2SO4 + 0.5N NaCl solution at room temperature. The results showed that when the ferrite network was discontinuous or globularized due to heat treatment, the SCC resistance was better than when the network was continuous. Welds deposited by the low heat input (GTAW) process showed better SCC resistance than their SAW counterparts because they had a finer ferrite network. The overall cracking was due to SCC in austenite and anodic dissolution of ferrite.

K.N. Krishnan

Papers

Effect of microstructure on stress corrosion cracking behaviour of austenitic stainless steel weld metals