Introduction
This report studies the macrostructure and microstructure of 38CrMoAl steel after hot rolled and tempering treatment. As a general typical steel materials, 38CrMoAl not only can be used in many fields, such as automobiles, locomotives, ships, aviation and other fields, but also can be used as the tank of coal and the exhaust pipes of diesel engine. Through the experiment of optical microscope, the carbon and grain size distribution of the 38CrMoAl steel samples were measured, and the microstructure was analyzed.
Experimental procedures
The samples in the experiment are 38CrMoAl steel produced by Nantong Machinery Plant. After the hot rolled, it was solution treatment at 943K for 1 hour, and then quenching in oil. The tower-free tempering was performed at 733K for 2 hours, and then the samples were section into 10×10mm×10mm for microscopic inspection.
Optical microscope analysis methods
The optical microscope (Olympus optical microscope SZX12) provided by Microstar laboratory was used to observe the microstructure of the sample. The raw material samples under the microscope were etched with a 5% nital. The sample was coated with gold so that the details of the image can be better observed. The microstructure of the sample is recorded by digital camera and the corresponding image analysis software(Image Pro Plus 9.0) is used to measure the grain size of sample.
Results and discussion
1.Macro observation
The macro appearance of 38CrMoAl steel is shown in Figure 1. Figure 1 shows that the sample is straight, no delamination and no obvious cracks. The surface of the sample is evenly distributed particles. It can be seen from the figure that the sample was well annealed and the hematite did not decompose.
Figure 1 macro observation of 38CrMoAl steel
2.Microscopic observations
Figure 2 is the microstructure observed under the optical microscope of 38CrMoAl steel after hot rolling and tempering. It can be seen from the figure that there are a lot of thick and thin grain boundaries distributed in the sample. The grain size distribution includes coarse grains, medium grains and fine grains. The average grain size is about 20μm.
Figure 2 Optical microscopy under the optical microscope of 38CrMoAl steel after hot rolling and tempering
3.Observation of grain size
The average grain size was measured by image analysis software as shown in Table 1. The average grain size of the sample after tempering is 18μm. It can be seen that the hot rolling and tempering treatment caused the grain size of the sample to decrease.
Table 1: grain size measurement data
Grain size (μm) Number of grains (%)
< 5 2.5
5-10 17
10-15 37
15-20 33
> 20 10
4. Analysis of microstructure
The microstructure of 38CrMoAl steel after hot-rolling and tempering, as shown in Figure 3. It can be seen from the figure that, due to the spheroidizing annealing, the pearlite microstructure of the sample is converted into a spheroidite structure, which is composed of a lot of evenly distributed spheroidite particles distributed in the matrix, and a small number of ferrite phases. The microstructure is refined and the mechanical properties are improved due to the hot rolling and tempering treatment.
Figure 3 Microstructure observed under the optical microscope of 38CrMoAl steel after hot rolling and tempering
Conclusion
In this report, the macro and micro observations of 38CrMoAl steel after hot rolling and tempering were studied. The results show that the macro structure of sample is good and the surface is even distributed particles. The microstructure shows that the grain size of sample after tempering is 18μm. The sample is mainly spheroidite and a small amount of ferrite is distributed in the matrix. Hot rolling and tempering treatment can effectively refine the grain size and improve the mechanical properties of the sample.
