Delayed cracking of low‐nickel austenitic stainless steel studied with constant load tensile testing

Abstract

A B S T R A C T Delayed cracking in unstable low-Ni austenitic stainless steel 204Cu was studied by constant load tensile testing. The developed testing arrangement enabled a systematical examination on the effect of applied stress, strain-induced α′-martensite and internal hydrogen content on time to fracture. Volume fraction of strain-induced α′-martensite was shown to affect cracking kinetics, except at a very high stress level. Hydrogen content had a marked effect on time to fracture, also at the highest applied stress level. When hydrogen content was reduced by annealing, delayed cracking kinetics and susceptibility were suppressed, and cracking required a considerably higher stress level. The apparent critical hydrogen content, below which delayed cracking was not observed, was about 0.85 wppm. According to scanning electron microscope and electron backscattering diffraction examination, fracture mechanism in the constant load test specimens was mainly transgranular quasi-cleavage, and cracking propagated along α′-martensite.Keywords constant load testing; hydrogen; hydrogen-induced delayed cracking; stainless steel; strain-induced α′-martensite; stress ratio.N O M E N C L A T U R E EBSD = electron backscattering diffraction LDR = limiting drawing ratio SFE = stacking fault energy TDS = thermal desorption spectroscopy I N T R O D U C T I O NDelayed cracking can occur in formed metallic components under static stress, applied or residual, below the yield strength of the material. It is a problem concerning high-strength steels and also some metastable austenitic stainless steels. Cracks may appear in successfully formed components after days or even months from forming, such as deep drawing. Metastable austenitic stainless steels, in which austenite is partly transformed to martensite under plastic strain, possess an excellent combination of strength and elongation, as the formation of strain-induced martensite increases work hardening of the material. Because of their high energy-absorbing capacity, metastable austenitic stainless steels have become attractive materials to be used, for example, in automotive crash-relevant structures. Nickel accounts for a significant percentage of the raw material cost of conventional austenitic stainless steels.Therefore, low-Ni austenitic stainless steels, in which nickel alloying is partly substituted with manganese and nitrogen, have become increasingly popular. They exhibit as good or even better mechanical properties than their nickel-alloyed counterparts, but are generally more susceptible to delayed cracking than the Fe-Cr-Ni austenitic stainless steels, if they experience strain-induced martensitic transformation during forming. This has been one of the important factors limiting increasing use of low-Ni austenitic stainless steels in many potential applications. Delayed cracking of metastable austenitic stainless steels is related to coexistence of solute hydrogen, straininduced α′-martensite and residual tensile stresses. 1-3 Crack initiation and growth are cont...

Hydrogen-Induced Delayed Cracking in TRIP-Aided Lean-Alloyed Ferritic-Austenitic Stainless Steels

“…Constant load tensile testing was conducted using a prior developed testing arrangement [ 27 ]. Specimens were first pre-strained to certain strain levels (0.27–0.31) to simulate the conditions after forming operations and to produce strain-induced α’-martensite in the material.…”

The influence of the tempering temperature on hydrogen embrittlement in carbonitrided modified SAE 10B22 steel

“…The hydrogen embrittlement is a failure caused by the action of hydrogen in combination with residual stresses present and/or applied, which leads to a reduction in plasticity, tenacity and strength capacity of the component [14,15]. The influence of hydrogen damage in metallic materials is being studied extensively in the literature [16][17][18][19][20][21][22][23][24][25][26].…”