Application of CNC tools in corrosion-resistant stainless steel parts

Abstract: This paper analyzes the quality problem of the stainless steel Z8CND17-04 material, and uses the metallographic method to analyze the fracture of the parts in the final fatigue test. It qualitatively explains that the hardened layer of the machined part is one of the reasons for the cracks on the surface of the parts. Combined with the company's existing production conditions, a cutting test plan is formulated to avoid the formation of the hardened layer on the surface of the parts and improve the quality and production efficiency of the parts.

Keywords: stainless steel; metallographic; hardness; tool life; hardened layer

1. Current situation and problem description

The material of the part is French standard grade Z8CND17-04, which belongs to martensitic stainless steel, with hardness HRC36-41 and surface roughness Ra0.4. It is difficult to realize general turning processing. The actual processing selects the blade DNGP15402KC730 of Kennametal Tool Company. Cutting speed 111 m/min; feed 0.02 mm/s; cutting depth 0.075 mm. After the part is processed, fatigue test is required. The test conditions are that the service life should reach more than 733,600 times under the action of cyclic loading with a minimum load of Fmin=-4859 KN and a maximum load of Fmax=4083 KN. However, the parts processed by the company broke and failed near the large end when the test reached 110,000 cycles.

2. Cause Analysis

2.1 Metallographic Analysis

Metallographic analysis shows that crack propagation is the main cause of this failure. The interface between the chrome-plated layer and the substrate can be seen from the cross section. The machined lines are wavy. The cracks in the chrome-plated layer extend into the substrate and are close to the fracture initiation source. The machined surface roughness is manifested as crests and troughs at the interface between the chrome-plated layer and the substrate. There is residual tensile stress at the crests and residual compressive stress at the troughs. Most of the cracks in the chrome-plated layer are generated at the crests of the interface, and there are extended cracks in the hardened layer in Figure 2. It is preliminarily determined that the extended cracks in the chrome-plated layer are caused by work hardening.

2.2 Comparative test analysis

The blades used by other suppliers are DNMG110404MF1025. We compared the cutting parameters, metallographic images after processing, and Vickers hardness, and the results are as follows:

From Table 1, it can be seen that the geometric radius, cutting speed, feed, and cutting depth of the blade of DNMG11040404MF1025 are 2 times that of DNGP150402KC730. And the interface between the chrome layer and the substrate is flat, and the difference between the center and the surface of the blade substrate is only 22 HV. However, the interface of our company is regularly wavy (consistent with the processing state of 0.02 mm/r), and most of the cracks in the chrome plating layer are generated at the wave crest of the interface, and the difference between the center and the surface of the substrate is 114HV (the hardening phenomenon of the processed surface can be reflected by detecting the difference in Vickers hardness between the surface and the center of the substrate). The blade has a small radius, and the cutting depth, feed and cutting speed are relatively low. When the cutting depth and feed rate are too small, the blunt part of the cutting edge of the tool will be squeezed more times per unit length of the machined surface, and the hardening phenomenon will also increase.

3. Process test

Combined with the company's existing production conditions, the work hardening phenomenon is improved by improving the tool geometry angle and cutting parameters. The test is as follows:

(1) Tool configuration: According to the analysis of the mechanism of cracks caused by work hardening, the front angle and back angle of the tool are increased in the test to keep the cutting edge sharp and reduce the work hardening phenomenon on the surface of the part.

(2) Cutting parameter configuration: According to the analysis of the mechanism of cracks caused by work hardening, too high or too low cutting speed will cause work hardening. Under the condition of meeting the requirements of part processing accuracy, we selected 9 groups of cutting parameter data for trial cutting, striving to find the most reasonable set of cutting parameters.

(3) Vickers hardness comparison: The difference in Vickers hardness between the surface and the core of the part processed by this set of blades DNGP150402KC730 is HV 114, indicating that the surface has hardened after processing. Such a hardening degree is unacceptable.
(4) Metallographic comparison: From the metallographic diagram, it can be seen that the hardened layer produced by the DNGP150402KC730 blade group is the thickest, the hardened layer produced by the DNMG110404MF1025 blade group is the thinnest, and the hardened layer produced by the CCGX120404ALH10 blade group is between the above two. It can be seen that the blade configuration and cutting parameter selection have a certain influence on the surface quality of the parts, especially for high-precision stainless steel parts, the substrate surface is prone to hardening. The hardening of the surface increases the surface brittleness, which can seriously lead to the generation of substrate cracks. As the cracks expand, the parts will fail due to fatigue fracture.
(5) Actual application effect: Taking various factors into consideration, the cutting conditions of item 7 in Table 2 were selected according to the actual situation to complete the processing of the parts.

4. Conclusion

Although the work-hardening layer on the surface of the parts will enhance its wear resistance, its brittleness will also increase at the same time, becoming the main reason for the occurrence of cracks and surface damage and fatigue failure. The surface residual stress of the parts can easily cause cracks on the surface and reduce the fatigue strength of the parts. From the vast number of tool brands, choosing a tool that is inexpensive and suitable for enterprise production and scientific research is a headache for manufacturing companies. The selection of tools is practical, reasonable and scientific, which can not only improve production and processing efficiency, but also maximize the processing potential of CNC processing equipment. At the same time, it can also greatly reduce the consumption of enterprise tools and achieve the purpose of saving tool costs. Therefore, in the case of a mixture of good and bad tools, tool selection becomes very critical. In the selection process, it is necessary to pay attention to cost performance, practice more, and compare more.