06 Cr19Ni10(SUS304) stainless steel M20 * 80 external hexagon bolts were found to have surface cracks during trial production, with a crack depth of about 0.30mm. The production process of this batch of bolts is: solid solution → drawing → cold heading → trimming → wire rolling → detection, and surface cracks were found during final detection.

1. Physical and chemical inspection
1.1 spectral analysis
The direct reading spectrometer was used for analysis, and its chemical composition met the standard requirements, as shown in Table 1.

1.2 metallographic examination
After polishing, more TiN inclusions were found to be distributed in chain and net-like aggregation. Non-metallic inclusions are assessed as: Class A 0.5, Class B 3.0, Class C 0, and Class D 1.0. After etching, it is found that there are more ferrite, which is distributed in chain, needle and network, and its microstructure is austenitic ferrite, and the grain size is 5. The α-phase area content was assessed as Grade 2.0, and the α-phase area content of the raw materials was determined to be Grade 2.0.
 

2. Analysis and discussion
Based on the above analysis, the chemical composition of the raw material meets the standard requirements, but in the raw material and bolt finished product, it is found that there are more than grade 2 α phase and TiN inclusions in chain and network distribution.

Formation and influence of 2.1 α phase
The formation and content of alpha phase in chromium-nickel austenitic stainless steel are related to various factors, such as chemical composition, heat treatment, cold and hot forming process, etc., and chemical composition is the decisive factor. Although the contents of this batch of bolt materials are within the range of standard requirements, the steel not only contains high residual strong ferrite forming element Mo, but also the content of austenite forming element Ni is close to the lower limit. The grade of α phase in steel decreases with the increase of nickel content and increases with the increase of chromium content.

For chromium-nickel austenitic stainless steel, in order to ensure that the structure of the steel after high temperature and rapid cooling is single-phase austenite, the content of nickel should be greater than the data given by the following empirical formula. Ni>1.1 ×(Cr + Mo +1.5 × Si +1.5 × Nb) − 30 × C- 0.5 × Mn-8.2. According to the alloy element content of this batch of materials, Ni>9.45% to ensure that a simple austenitic tissue is obtained, while the nickel content in this batch of materials is lower, the Ni equivalent is low and the tissue appears δ ferrite.

The area content of α phase in the material is calculated according to theory: α phase level = 1.0 × Si 0.5 × Cr 1.64 × Ti-10.86 × C- 0.29 × Ni-0.08 × Mn-4.64 = 2.399~2.4, which is consistent with the measured value. This shows that the appearance of more alpha phase in the material is due to the unreasonable ratio of alloying elements, the low nickel content and nickel equivalent, and the high residual strong ferrite forming elements.

There are differences in chemical composition, mechanical properties and thermal stability between the ferrite phase and the austenitic matrix, and the deformation flow performance is also different during cold deformation, so the appearance of ferrite brings many adverse effects to austenitic stainless steel, especially its plasticity is significantly reduced, while the tendency of cracking in cold and hot processing increases.