A Brief Analysis of Performance Differences Between 35CrMo and 4145H Quenched and Tempered Forged Bars

Abstract:

This study compares the mechanical properties of 35CrMo and 4145H, two types of medium-carbon alloy structural steels, using Φ200mm forged bars of the same specifications subjected to quenching and tempering treatments. The focus was on the mechanical properties at R/2. Results show that compared to 35CrMo, the yield strength of the 4145H quenched and tempered sample increased by approximately 39% to 965 MPa, and the tensile strength increased by approximately 29% to 1118 MPa. The elongation decreased by about 15.4% to 14.8%, and the reduction of area decreased by about 11% to 57%. The impact energy increased by about 73% to 75 J. The section hardness results also confirmed that 4145H has superior hardenability.

Keywords:

Quenching and Tempering, Forged Bar, Mechanical Properties

In recent years, with the development of China’s social economy, the domestic production rate of various types of machinery equipment has increased, leading to higher requirements for material performance. Medium-carbon alloy structural steels such as 35CrMo and 4145H possess high static strength, impact toughness, and good hardenability, making them widely used in petroleum machinery equipment for manufacturing critical parts subject to impact, bending, torsion, and high loads. They are among the commonly used high-strength steels in the petroleum industry.

Factors affecting the mechanical properties of this type of alloy steel include chemical composition, forging, and heat treatment. In addition, size specifications play a key role. Currently, there is a lack of actual measurement data for large specification forged bars in data manuals or other research studies. Therefore, comparative studies on the mechanical properties of large specification forged bars made of 35CrMo and 4145H under the same size conditions are even rarer. To understand the actual mechanical properties of parts during product design or material selection, it is crucial to evaluate the material performance at the worst-performing sections of the components.

1. Test Materials

The test materials were manufactured using an electric furnace + ladle refining + vacuum degassing process. Both 35CrMo and 4145H forged round bars with dimensions of Φ200mm × 300mm long were used, one bar per material. The chemical composition of 35CrMo adheres to the GB/T 3077-2015 standard for alloy structural steels.

2. Heat Treatment Process for Forged Round Bars

For medium-carbon alloy steels, quenching and tempering are commonly used. Quenching involves heating the steel above the critical temperature and then cooling it rapidly to transform austenite into martensite. Tempering follows to improve the comprehensive mechanical properties. For both 35CrMo and 4145H steels, which have carbon contents between 0.3% and 0.50%, the quenching temperatures range from AC3 + (30 to 50) °C. Based on the characteristics of each material and practical experience, the forged bars were treated in a multi-purpose furnace following the specified parameters:

  • 35CrMo: Quenching temperature 860°C, cooled using PAG aqueous quenching medium; tempering at 580°C, water-cooled.
  • 4145H: Quenching temperature 845°C, cooled using PAG aqueous quenching medium; tempering at 560°C, water-cooled.

3. Sampling and Testing Methods

Sampling was conducted according to GB/T 2975 requirements after quenching and tempering of the forged bars. Tensile tests were performed using a CMT-5105 microcomputer-controlled universal testing machine following GB/T 228.1-2010 standards. Impact tests were conducted using a B-300W microcomputer-controlled semi-automatic impact testing machine according to GB/T 229-2020 standards. Hardness tests were performed using a Rockwell hardness tester following GB/T 230.1-2009 standards. After grinding and polishing, the samples were etched with a 4% nitric acid alcohol solution and observed under a Leica DM1750M metallographic microscope according to GB/T 13298-2015 standards. Actual grain size was determined using the bitter acid method following GB/T 6394-2017 standards.

4. Results and Analysis

After quenching and tempering, the mechanical properties of the Φ200mm forged bars of different materials are summarized in Table 1, comparing the average of two sets of measured data:

  • Tensile Properties Difference: The tensile strength and yield strength of 4145H were 1118 MPa and 965 MPa, respectively, which is 29% and 39% higher than those of 35CrMo (868 MPa and 695 MPa). The elongation of 4145H was 14.8%, about 15.4% lower than that of 35CrMo. The reduction of area was 57%, about 11% lower than that of 35CrMo. The main reason for these differences lies in the variations in C content and the alloying elements Mn, Ni, and Cr.
  • Impact Properties Difference: The impact energy of 4145H (75 J) was significantly higher (about 73%) than that of 35CrMo (44 J). This is mainly due to the difference in Ni content, which enhances both strength and toughness while lowering the transition temperature from brittle to ductile behavior.
  • Hardness Difference: Within the same surface hardness range of 31-36 HRC, the hardness at a depth of 35mm from the surface for 4145H reached 35 HRC, whereas the average hardness for 35CrMo was only 25 HRC. This indicates that the hardness of 4145H at R/2 is almost consistent with the surface hardness (36 HRC), confirming its superior hardenability compared to 35CrMo.

5. Conclusion

  • Through tensile and impact tests on quenched and tempered forged bars under the same specifications and within the same hardness range (31～36 HRC), the tensile and yield strengths of 4145H at R/2 were 29% and 39% higher than those of 35CrMo, respectively. The impact energy was approximately 73% higher than that of 35CrMo.
  • By comparing the hardness differences between the surface and a depth of 50mm from the surface, it was found that the hardness decrease in 4145H was significantly smaller than that in 35CrMo, further verifying that 4145H possesses superior hardenability compared to 35CrMo.