Stress Intensity Range Dependent Slowing Down of Fatigue Crack Growth under Strain‐Induced Martensitic Transformation of Film‐Like Retained Austenite

• Published in: Steel research international Vol. 95; no. 4
• Main Authors: Kumar, Gaurav, Ghosh, Sumit, Pallaspuro, Sakari, Somani, Mahesh C., Kömi, Jukka, Mishra, Sushil K., Gokhale, Amol A
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.04.2024
• Subjects: advanced high‐strength steel; Crack propagation; Direct quenching; fatigue crack growth; Fatigue cracks; Fatigue failure; Fracture mechanics; Martensite; Martensitic stainless steels; Martensitic transformations; Metal fatigue; Phase transitions; Retained austenite; retained austenites; Steel; Strain; Stress intensity; stress intensity ranges
• ISSN: 1611-3683; 1869-344X
• DOI: 10.1002/srin.202300578

A clear understanding of strain‐induced martensitic transformation of filmy retained austenite (RA) near stable cracks is required. Literature shows RA transformation during fatigue crack growth (FCG) in steels containing blocky but not film‐like RA, as the latter is known to resist strain‐induced phase transformation. This work investigates the transformation in 0.2% C steel processed by direct quenching and partitioning (DQP) containing filmy and a small vol% of blocky RA and compares it with RA‐free direct quenched (DQ) steel. While the DQ steel is lath‐martensitic, DQP steel has 8.5 vol% film‐like RA evenly distributed between martensite laths. The experimental FCG rates are comparable for both steels in the low ΔK$\Delta K$ regime but increasingly differing in the Paris regime, the Paris law exponent being lower for DQP (n = 2.1) compared to DQ (n = 2.5), confirming that resistance to FCG is dependent on the stress intensity range (ΔK$\Delta K$). Moreover, RA content near crack surface decreases during crack growth from 7.5 vol% at ΔK$\Delta K$ of 25 MPa√m to 2.3 vol% at ΔK$\Delta K$ of 54 MPa√m. Results show that higher the stress intensity range in steels with film‐like RA, the higher the degree of RA transformation and more suppression of FCG rate. Fatigue crack growth rate as well as the slope of the crack growth rate versus ΔK for 0.2%C steel with martensite and 7.5% (blocky + filmy) retained austenite are lower than for martensite. Increasing extent of stain‐induced martensitic transformation occurs with increasing ΔK$$ \Delta K$$. Blocky austenite transforms completely at low ΔK$\Delta K$, while filmy austenite transforms gradually with increasing ΔK$\Delta K$ till catastrophic fracture.