Effect of multi-stage heat treatment regime parameters on the structure and mechanical properties of modified 35HML chromium-molybdenum steel

Abstract

This paper presents the results of a comprehensive experimental study of the effect of multi-stage heat treatment regime parameters on the structure and mechanical properties of modified cast chromium-molybdenum steel 35HМЛ, used in the manufacture of cast parts for mining excavators at JSC “Almalyk Mining and Metallurgical Combine”. The melt was modified with a complex Fe-V-Al-Ca ferroalloy composition (V 25–30 %, Al 15–20 %, Ca 5–10 %, balance Fe) in an amount of 0.15–0.18 wt. %, introduced into the ladle onto the metal stream at a temperature not below 1520 °C. A four-stage heat treatment scheme was developed: homogenizing annealing at 880 °C → normalization at 880 °C → quenching from 850 °C in preheated water at 60 °C → high tempering at 600–650 °C. The choice of preheated water at 60 °C as the quenching medium is experimentally justified by comparison with industrial oil И-20А: both media provide practically identical hardness (62 HRC versus 61.5 HRC), while preheated water reduces the proportion of castings rejected due to cracks from 12 % to 2 %. The dependence of hardness on tempering temperature in the range of 450–650 °C was established, and the recommended range of 600–650 °C was justified, excluding the zone of type II temper embrittlement (450–550 °C). The developed technology was implemented at the Central Repair and Mechanical Plant (CRMZ) of JSC “AMMC” (Implementation Certificate No. SL-355/1 dated April 16, 2025), providing a 35 % increase in Charpy impact toughness (from 540 ± 22 to 730 ± 31 kJ/m²) and an increase in the service life of ЭКГ-10 excavator support wheels from 2800 to 4200 hours.