Abstract
The study presents the effect of annealing process parameters on the microstructure, hardness, and strain-hardening coefficients, that is, the strength coefficient c and the strain-hardening exponent n, of 42CrMo4 steel. Seven selected annealing time–temperature schemes are examined for superior steel formability in cold metal forming conditions. The c and n coefficients are first determined in experimental upsetting of annealed samples and then used in FEM (finite element method) simulations of the upsetting process. The results demonstrate that the strain-hardening coefficients (c and n) depend on the employed annealing scheme. Compared with the as-received sample, the annealing process reduces the true stress and effectively decrease the hardness of 42CrMo4 steel; improves microstructural spheroidization; and, consequently, facilitates deformability of this material. The annealing schemes, relying on heating the material to 750 °C and its subsequent slow cooling, lead to the highest decrease in hardness ranging from 162 to 168 HV. The results obtained with the SEM-EDS (scanning electron microscopy-energy dispersive spectrometer), LOM (light optical microscopy), and XRD (X-ray diffraction) methods lead to the conclusion that the employed heat treatment schemes cause the initial ferritic-pearlitic microstructure to develop granular and semi-lamellar precipitation of cementite enriched with Mo and Cr in the ferrite matrix. In addition, the annealing process affects the growth of α-Fe grains. The highest cold hardening rate, and thus formability, is obtained for the annealing scheme producing the lowest hardness. The results of FEM simulations are positively validated by experimental results. The obtained results are crucial for further numerical simulations and experimental research connected with developing new cold metal forming methods for producing parts made of 42CrMo4 steel.
Highlights
Low-alloy structural steels are widely used in mechanical, automotive, mining, and machine building industries and undergo shaping by metal forming, machining, welding, surfacing, and many other techniques [1,2,3,4,5]
The initial hypoeutectoid steel [8,40,41]. It can be observed in the dark-field image of as-received 42CrMo4 coupon has a pearlite-ferrite microstructure (Figure 2) that is typical of the metallographic sample (Figure 2d) that the cementite is visualised hypoeutectoid steel [8,40,41]
This study investigated the effects of annealing time and temperature on the microstructure, hardness, and strain-hardening coefficients of low-alloy structural steel grade 42CrMo4 (AISI 4140)
Summary
Low-alloy structural steels are widely used in mechanical, automotive, mining, and machine building industries and undergo shaping by metal forming, machining, welding, surfacing, and many other techniques [1,2,3,4,5]. Annealing time–temperature parameters, like in other heat treatments, are affected by the chemical composition of steel, but they depend on the combination of metal treatment history (primary microstructure), initial plastic deformation, part dimensions, and many other factors affecting the properties of heat-treated 42CrMo4 steel [1,8,9,10,11,12]. Despite the fact that the literature of the subject [8,9] reports general parameters of annealing, still, the best solution for obtaining superior deformability of specific steels is to conduct an experimental investigation of time–temperature parameters of this heat treatment
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