In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

Abstract

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) tests cycling from 400°C to 700°C under full reverse strain-controlled condition were investigated. Stress-mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high-temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi-cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter. KEYWORDS cyclic softening, cyclic stress-strain behavior, fatigue life, hot work die steel, thermomechanical fatigue | INTRODUCTIONDuring the die filling process, like die casting, hot extrusion, hot forging or hot stamping, the service life of hot work die is limited because of their extreme working environments in terms of the cyclic alternating high temperature, which results from the close contact between hot work die and hot work piece (1200°C for steel forging) or molten metal (675°C for Al-alloy casting, 930°C for Cu-alloy casting) and mechanical stress, leading to cyclic plastic deformation, which gives rise to crack initiation. [1][2][3][4] All along, the hot work die is damaged through a complex oxidation/wear/non-isothermal low cycle fatigue interaction. 5,6 Therefore, non-isothermal low cycle fatigue damage (occurrence of the cyclic reversed plasticity at low and high temperatures) of hot work die is usually called thermomechanical fatigue (TMF). According to incomplete statistics 7 , thermal cracking caused by TMF accounts for approximately 80% of the total failure die-casting dies. Thus, good TMF behavior can prolong the service life of the hot work die. However, the literatures about TMF behavior of hot work die steel are rarely found. 5,6,8,9 As TMF tests are time consuming and require expensive equipment, the fatigue life prediction with varying Nomenclature: ϕ, mechanical strain/temperature true phase angle; Δε mech , mechanical strain range; Δε mech /2, mechanical strain amplitude; R ε , strain ratio; (ε mech ) max , maximum mechanical strain; (ε mech ) min , minimum mechanical strain; T 0 , reference temperature; T max , maximum temperature; T min , minimum temperature; σ max , maximum stress; σ min , minimum stress; σ m , mean stress; Δσ, st...

Material behaviour of a dual hardening steel under thermomechanical loading

“…Furthermore, many cracks with different lengths are visible in the specimens TMF_02 and TMF_08. In Zuo et al, 12 it was shown that under TMF loading, a great number of cracks are visible, which connects with the observations in the work.…”

“…With higher stress amplitudes, the crack behaviour changes from intercrystalline to transcrystalline. Another hot-work steel was investigated under strain-controlled IP and out-of-phase (OP) TMF tests in Zuo et al 12 The investigations of the 4Cr5MoSiV1 steel showed that the lifetime under OP TMF is lower than IP TMF and the cracks, which occur, are fewer but larger at IP TMF tests than OP TMF tests.…”

A Temperature-Dependent Viscoplasticity Model for the Hot Work Steel X38CrMoV5-3, Including Thermal and Cyclic Softening under Thermomechanical Fatigue Loading

“…A reduction of hardness from initially 47 to 33 HRC was observed in [19] in a superficial region with a depth of 0.3 mm of a hot forging tool made from martensitic steel X40CrMoV5-1, being a preferred site for the nucleation and propagation of fatigue cracks. Continuous softening of the hot work steel 4Cr5MoSiV1 was observed in [20] in strain-controlled in-phase (IP) and out-of-phase (OP) TMF tests in the temperature range from 400 to 700 • C. Due to softening, the stress levels decrease in the entire temperature range with an increasing number of cycles. Softening in the martensitic steels is attributed to two mechanisms, namely thermal softening controlled by temperature, e.g., analyzed in [21][22][23], and cyclic softening controlled by cyclic plasticity, e.g., analyzed in [24] and in the review article by Jilg [25].…”