Analysis of Fracture on 35CrMo Steel (870℃ Normalized Treatment)
Abstract
This paper aims to analyze the fracture of 35CrMo steel with 870℃ normalizing treatment. The microstructure and composition of the steel and the results of the mechanical properties test of tensile strength and impact toughness are carried out by metallographic observation and spectrochemical analysis, respectively. The fractures of the specimen were observed under SEM microscope, and the fracture and surface morphology were obtained. In addition, in order to further analyze the causes of the fracture, the commonly used fracture analysis method, such as microhardness analysis, crack propagation analysis, strain aging analysis and ferrite / martensite analysis were respectively used to analyze the specimen surface, interface and adjacent area of the fracture. Finally, the causes of the fracture are discussed.
Keywords: 35CrMo steel; fracture analysis; normalizing treatment
1 Introduction
35CrMo steel is a kind of alloy structural steel, which is one of the most widely used quenching and high temperature tempering steel. It belongs to Cr-Mo system, and its performance indexes are better than those of 25CrMo steel. Therefore, in the engineering, it is usually used to make all kinds of important machining parts, including the parts with high intensity, large section size and many welding structures. The normalizing treatment temperature of the alloy structural steel is about 840℃ ~ 870℃, and the normalizing treatment of the 35CrMo steel is 870℃ in this paper.
2 Experiment
2.1 Specimen preparation
The [35CrMo] steel used in this experiment was processed by the forging and rolling to the specification of d50, and then normalized and cooled to the room temperature in the air. After that, the metallographic observation, spectrochemical analysis, tensile strength test and impact toughness test were all conducted.
2.2 Metallographic examination and spectrochemical analysis
The metallographic examination and spectrochemical analysis of the steel were performed according to the national standard to obtain the microscopic morphology and chemical composition of the steel.
2.3 Tensile test and impact toughness test
The strength and impact toughness of the specimens were tested on the universal testing machine. The test temperature was room temperature and the test speed was 100mm/min.
2.4 Fracture analysis
The fractured specimen was mounted on the sample holder, and a suitable amount of gold-plated platinum was sputtered on the surface of the fracture to make the fracture more clear. Then, SEM microscope was used to observe the fracture and obtain the fracture morphology, interface microstructure and adjacent area microstructure. Additionally, microhardness analysis, crack propagation analysis, strain aging analysis and ferrite / martensite analysis were adopted to analyze the specimen surface, interface and adjacent area of the fracture, respectively.
3 Results and discussion
3.1 Metallographic examination results
The metallographic examination results showed that the as-forged steel contained γ phases with blocky structure, and the acicular ferrite with flaky structure was evenly distributed on the grain boundary and within β phase.
3.2 Spectrochemical analysis results
According to the results of spectrochemical analysis, the chemical composition of the sample was determined to be 0.35%~0.41% of C, 0.28~0.35%Si, 0.80~1.0%Mn, 0.90~1.1%Cr and 0.20~0.30%Mo, respectively.
3.3 Mechanical properties
The results of the mechanical properties test showed that the tension strength of the specimen was 686.2MPa, and the impact toughness value was 32J/cm2.
3.4 Fracture analysis results
The observation results showed that the fracture of the specimen was dimple-dimpling fracture with dimple fracture on the surface, and a large number of cleavage fracture in the dimples. In addition, a large number of globular grains were found in the fracture interface and adjacent area of the fracture. When different fractured methods and fracture surfaces were studied exhaustively, it showed that the macro dimple fracture was consistent with the micro-fracture in the dimple fracture. There are cleavage fracture, ductile fracture and micro-cleavage fracture on the cleavage fracture surface, which indicates the mixed fracture mode of dimple and cleavage fracture.
The microhardness analysis showed that the average microhardness value of the cleavage fracture was larger than that of the dimple fracture, which showed that the tensile stress was greater in the cleavage fracture area. It showed that the cleavage fracture was directly related to the large stress concentration.
The ferrite/martensite analysis also showed that the ferrite-martensite ratio in the cleavage fracture area was greater than that in the dimple fracture area, indicating that higher carbon content in the cleavage fracture area would cause local softening and easy to fracture. Besides, the results of the crack propagation test and strain aging test indicated that the fracture surface was mainly caused by the dynamic fatigue crack, while the effects of static fatigue and stress corrosion were relatively low.
4 Conclusion
The static and dynamic fracture of 35CrMo steel with 870℃ normalizing treatment was analyzed. The results showed that the fracture of the sample was mainly caused by the dynamic fatigue crack, while the effects of static fatigue and stress corrosion were relatively low. These conclusions have certain guiding significance for the material selection, heat treatment, processing and application of 35CrMo steel.
