High-Temperature Precipitation Design-of-Experiments Simulation in Low-Alloy Cr–Mo–Ni Hot Forging Steel

Roberto Gonzalez-Ojeda,Jose Manuel Sanchez-Moreno,Gonzalo Gonzalez-Reyes,Octavio Lozada-Flores

doi:10.3390/met11071054

Roberto Gonzalez-Ojeda, Jose Manuel Sanchez-Moreno + Show 2 more

Open Access

https://doi.org/10.3390/met11071054

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Abstract

The role of alloying elements such as Cr, Mo and Mn on low-alloy 8620 steel during hot forging operations is not yet clear, as, during deformation in the 1000~1100 °C temperature range, the austenite grain size remains small, ensuring the capacity of the forged part to be subsequently modified by surface hardening procedures. This work analyzed a deformed bar considering hardness at different geometry zones, along with SEM and TEM microstructures of previous austenite grains and lamellar martensite spacing. Moreover, Thermocalc simulations of M7C3, M23C6 and MnS precipitation were combined with Design of Experiments (DOE) in order to detect the sensitivity and significant variables. The values of the alloying elements’ percentages were drastically modified, as nominal values did not produce precipitation, and segregation at the austenite matrix may have been responsible for short-term, nanometric precipitates producing grain growth inhibition.

Highlights

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The use of low-alloy steels in hot forging operations is indispensable for the automotive industry, as the low cost of materials used for transmission gears and other structural parts depends on reliable steels that are capable of surface modification [1]
In the case of 8620 Ni–Cr–Mo low-alloy steel, high-temperature deformation in the 1000~1100 ◦ C range is performed using an industrial die press in short periods of time after the steel bar is removed from the furnace [2]

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Simulations were carried out using the Thermocalc TC-PRISMA precipitation module by feeding the program with the corresponding amount (weight %) of the alloying element, temperature (either 1000 or 1100 ◦ C) or an austenite iron FCC matrix, and tracing the precipitation of the M7 C3 , M23 C6 or MnS phases in a 1000 s period, as hot forging manufacturing operations are performed at high speed many times in a few seconds for various deformation steps [1]. The statistical results were considered adequate for this analysis and focused on the effect of each factor on the precipitation of specific particles

Results and Discussion

Micrograph

Lamellar

14. Thermal