Stress Oriented Delayed Cracking Induced by Dynamic Martensitic Transformation in Meta‐Stable Austenitic Stainless Steels

Abstract

Delayed cracking in meta‐stable austenitic stainless steels AISI 304 and AISI 301 has been investigated by deep drawing test in this study. Specimens from both steels were deformed at different drawing ratios, forming temperatures and hydrogen pre‐charged conditions. The strain, martensitic fraction and residual stress distributions were characterised on the deep drawn specimens. Their influences on delayed cracking behaviours were quantitatively studied. We found that the incubation time to fracture and the fracture length depend chiefly on the amount of strain induced phase transformation and transformation induced residual stresses when the materials contain ∼1ppm hydrogen. When the materials were electrochemically charged with hydrogen up to the amount of 20ppm, the incubation time to fracture declined with increasing hydrogen contents following a power law relation. Besides, the fracture mode turned from a ductile nature to trans‐granular and later on to inter‐granular fracture feature with increasing hydrogen contents.

The TRIP Effect and Its Application in Cold Formable Sheet Steels

“…The Transformation‐Induced‐Plasticity (TRIP) effect in steels denotes the significant enhancement of both strength and ductility through austenite to martensite transformation during the plastic deformation . The transformation of (retained) austenite to martensite during deformation at or close to room temperature develops a high carbon martensite phase, which substantially raises the strain‐hardening rate and strengthens the material effectively, as shown in Figure . The progressive transformation of austenite to martensite during straining is visible in the serrations and leads to high strain hardening and postponed occurring of necking.…”

Delayed Cracking of Metastable Austenitic Stainless Steels after Deep Drawing

“…18) Parameters of hydrogen transport, e.g. diffusivity and permeability, in stainless steels at room temperature have been found to be relatively insensitive to chemical composition.…”

A Continuum Model for the Effect of Dynamic Recrystallization on the Stress–Strain Response

“…In the mass-manufacturing industry there is an ongoing drive to improve product quality or reduce costs by reducing finishing steps. For e.g., automotive or consumer products made from austenitic stainless steel or High-Strength Low-Alloy steel (HSLA) there is a clear need to reduce springback, increase formability or decrease the amount of residual stress after forming [ 1 , 2 ]. To achieve these goals, one option is to apply a hot forming process where the material is processed at temperatures between 900 C and 1100 C. At these temperatures the yield stress of the material is considerably lowered.…”