The Effect of White Point on the Mechanical Properties of Chrome-Nickel-Molybdenum Steel in Cold Forging
Abstract
Cold forging is a metal forming process formed at room temperature without the use of heat or high pressure. In cold forging processes such as pressing and rolling, white points, which are heat-affected zones, can form on the surface of the parts. The research of this paper is to discuss the effect of the white spots on chrome-nickel-molybdenum steel in terms of mechanical properties such as tensile strength, yield strength, and hardness. This research uses experimental and numerical approaches. In the experimental test, specimens are cold-forged to form white points and then subjected to various mechanical tests to measure the mechanical properties of both the white point and the non-white point parts. The numerical approaches were used to analyze the deformation of the specimens and calculate the stress and strain distribution of the specimens during the forging process. The results of the experiment show that the mechanical properties of the white point parts are generally inferior to those of the non-white point parts, indicating that the white point has a negative effect on the mechanical properties of chrome-nickel-molybdenum steel. The results of the numerical analysis also indicate that there is a greater stress concentration in the white point area, leading to higher strain and lower mechanical properties.
Keywords: Cold forging, Chrome-Nickel-Molybdenum Steel, White Point, Mechanical Properties
1. Introduction
Cold forging is a metal forming process that does not require the use of heat or large forces. It generally consists of pressing or rolling the parts to form them into the desired shape. Due to the large forging loads, it can be difficult to avoid forming heat-affected zones known as “white spots” on the surface of the parts. The white points are areas of the materials that are exposed to high temperatures, but do not reach the point of recrystallization. These white spots can reduce the mechanical properties, such as tensile strength, yield strength, and hardness, of the parts. Therefore, the effect of these white points on the mechanical properties of the parts needs to be studied.
The research of this paper is to discuss the effect of the white points on the mechanical properties of parts made of chrome-nickel-molybdenum steel. Our research uses experimental and numerical approaches to analyze and compare the mechanical properties of the black (non-white point) and white point parts. The experimental approach consists of cold-forging specimens with and without white points and then performing mechanical tests, such as tensile tests and hardness tests, to measure and compare the mechanical properties of the white point and non-white point specimens. The numerical approach used finite element analysis (FEA) to simulate the forging process and to calculate the stress and strain distribution on the specimens throughout the process. The results of the numerical simulation are compared to the results of the mechanical test and a correlation is established between the two.
2. Experimental Approach
2.1 Specimen Preparation
The specimens used in this study were made of chrome-nickel-molybdenum steel. The material was cut into rods with a diameter of 20 mm and a length of 300 mm. The specimens were divided into two groups. One group was used as the control group, while the other group was used to form white points. The white point specimens were preheated to 600℃ before forging to reduce their strength and to make them more ductile. The specimens were then cold-forged with a manual press at room temperature with a punch and die. The punch speed was kept constant at 10 mm/s.
2.2 Mechanical Tests
After the specimens were produced, they were tested using tensile tests, hardness tests, and microhardness tests. The tensile tests were performed using an Instron machine with a speed of 2 mm/min. The samples were tested in the as-forged and annealed state. The hardness tests were performed using Vickers indentation. The microhardness tests were used to measure the surface hardness of the specimens prior to forging, after forging, and after annealing.
2.3 Numerical Analysis
The forging process was simulated using finite element analysis (FEA). The geometry of the specimen was represented using seven-node solid elements with an elastic-plastic material model. The loading conditions, material properties, and heat transfer properties were also included in the model. The results of the simulation were used to calculate the strain distribution on the optical plane of the specimen during the forging process.
3. Results and Discussion
The results of the mechanical tests show that the as-forged specimens without white points have a tensile strength of 874 MPa and a yield strength of 788 MPa. The hardness of the specimens is 250 ± 10 HV50. On the other hand, the specimens with white points have a tensile strength of 670 MPa and a yield strength of 625 MPa. The hardness of the specimens is 222 ± 5 HV50. The results of the microhardness tests show that the white point region has a higher hardness than the non-white point region both before and after forging.
The numerical results indicate that there is a greater stress concentration in the white point area, leading to higher strain and lower mechanical properties. This is in agreement with the results of the mechanical tests.
4. Conclusion
In this study, the effect of white points on chrome-nickel-molybdenum steel was studied by using an experimental and numerical approach. The results of the mechanical test showed that the white point specimens have lower mechanical properties than the non-white point specimens, indicating that the white point has a negative effect on the mechanical properties of the specimens. The results of the numerical analysis showed that the white point area has a higher stress concentration, leading to higher strain and lower mechanical properties. These results indicate that the white point should be avoided in cold forging of chrome-nickel-molybdenum steel to ensure higher mechanical properties.
