Role of residual stresses in the evaluation of SCC susceptibility of welded joints was studied on the butt-welded stainless steel specimens. Distribution of residual stresses was measured by both nondestructive X-ray method and modified Kelsey's method which was newly proposed for the determination of residual stresses within the specimens. Comparison of data accuracy and measuring facilities in both methods was made and resulted that modified Kelsey's method was an available one. Threshold residual tensile stress which causes SCC without applied stresses was found to be 11kg/mm² in the combination of austenitic stainless steel SUS 304-42% boiling magnesium chloride solution. However, in some cases, cracks initiated at the place with surface residual stresses lower than the threshold residual stress, therefore materials properties, such as pre-strain (defect concentration), surface roughness and residual stresses within the specimen also should be taken into account as well as the surface residual stresses. For the evaluation of SCC susceptibility in the test case where applied stresses were imposed on the specimens with residual stresses. Resultant stress composed of applied and residual stress at crack initiation was proposed and found to be an effective parameter in most cases of all specimens tested. But this parameter also becomes improper one for the estimation of SCC susceptibility, especially in the case of specimens which have both hardened surface and therefore higher residual stresses than the original yield strength. Shot peened and ground specimens are this case. The reason for this inconvenience and other troubling cases in using resultant stress are discussed based on the materials properties of specimens.

View full abstract

Anodic polarization curves and the susceptibility to stress corrosion cracking (SCC) have been studied for Haynes Alloy No. 25 (0.1% C-20% Cr-15% W-10% Ni-bal. Co) in aged (TP-1 specimen) or overaged (TP-2 specimen) conditions. The effects of overaging heat treatment of the aged specimen (TP-1) on the anodic polarization curves and SCC susceptibility were examined, and the relation between the electrochemical behavior and the susceptibility to SCC has been discussed. The results obtained are as follows: (1) The aged specimen (TP-1) has lower corrosion rates than that the overaged specimen (TP-2) in boiling 1mol/l H₂SO₄ solution. (2) The polarization behavior in 30% MgCl₂ solution at 80°C was greatly affected by aging condition. The aged specimen (TP-1) exhibited active-passive behavior with low passive current densities in contrast to totally active dissolution of the overaged specimen (TP-2). (3) Transgranular stress corrosion cracking was found in C-ring aged specimens (TP-1) immersed in boiling 42% MgCl₂ solution, while no cracking was found in overaged specimens (TP-2) without passivity in MgCl₂ solution. (4) Overaging treatment of previously cold worked aged specimen (TP-1)at 800°C for more than 0.1h resulted in disappearance of a passive region in polarization curve and no SCC in MgCl₂ solutions. These overaging effects can be explained on the basis of the relationship between the preferential precipitation of Cr carbides at slip lines and the concentrations of metallic Cr and W required for passivation in matrix.

View full abstract

The variation of threshold stress to SCC with Ni content of 25 Cr-0-9 Ni steels has been studied by constant load SCC test in 45% MgCl₂ solution boiling at 427K. The lowest value is obtained at a content of about 2% Ni and the highest one at 6-8% Ni where the α-phase in the steel attains 40-50% in volume ratio. To consider the reason of high resistance of duplex stainless steels, WOL test, constant load test and electrochemical tests have been conducted using 25 Cr-6 Ni (α+γ) steel as well as 28 Cr-4 Ni (α) and 21 Cr-9 Ni (γ) steel which are equivalent on composition respectively to α- and γ-phases of the duplex steel determined by EPMA analysis. The duplex steel has higher value of K_ISCC and lower rate of crack propagation than those of any steel consisted of single phase. The cracks of duplex steels have been observed to propagate across α-phase and to be retarded by γ-phase. Electrochemical coupling of the γ-steel with the γ-steel while holding at each proof load makes no significant effects on their times to failure. The high SCC threshold stress of duplex stainless steels could be attributed to a keying effect of the γ-phase which is caused by mechanical effects based on mixed (α+γ) duplex structure rather than an electrochemical one.

View full abstract

Effect of cathodic hydrogen charging on crack growth in austenitic stainless steels under alternating loading of a low frequency was investigated in 0.5M H₂SO₄ solution with and without a poison (P+CS₂) at ambient temperature. The fatigue crack growth rates under both open circuit and catholic charging decreased with an increase in Ni equivalent, namely in the order-304L>316>310S. The fatigue crack growth rate in austenitic stainless steels under cathodic charging became greater than that in open circuit, and this acceleration of crack growth influenced by hydrogen was remarkable in lower Ni equivalent 304L steel.

View full abstract

The degree of sensitization was determined for sensitized stainless steel specimens by means of the EPR (Electrochemical Potentiokinetic Reactivation) measurement. Their susceptibility to intergranular stress corrosion cracking (IGSCC) was also evaluated by means of CBB (Creviced Bent Beam) test in simulated BWR environment.
A good correlation was observed between the activation ratio, R_a (=maximum activation current/critical passivation current), in the EPR measurement and the maximum crack depth in the CBB test. The EPR measurement was qualified to be able to used for screening the IGSCC susceptibility of material in simulated BWR environment. The screening criterion of R_a=1% was determined on the basis of the high reliability and the good screening ratio.

View full abstract

SCC-Potential-pH diagrams of SUS 304 and SUS 316 stainless steels have been constructed in boiling 20% NaCl, and 44.2% CaCl₂ solutions and in high temperature water with 500ppm Cl^- at 523K. SCC-Potential-Concentration diagrams of both stainless steels have been also made in MgCl₂ solutions at 353K.

View full abstract