06 Cr19Ni10(SUS304) stainless steel bolt surface crack analysis

06 Cr19Ni10(SUS304) stainless steel bolt surface crack analysis

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.

Table 1 Chemical composition of bolt (mass fraction,%)

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.
 

Formation and influence of 2.2 TiN inclusions

TiN inclusions are the main inclusions in Ti-containing stainless steel, and their formation and distribution are mainly related to the casting process and method, and secondly to the Al-containing steel. If no effective protective pouring measures are taken, aluminum is easily oxidized to form Al.₂O₃inclusions, and become the core of TiN nucleation. TiN inclusions grow up during the floating process, which is easier to Al.₂O₃Inclusions form an aggregate distribution of inclusion clusters.
 

The purpose of adding Ti to steel is to inhibit (Cr,Fe)₂₃C₆Precipitation on grain boundaries to eliminate intergranular corrosion tendency. When Ti element precipitates in large quantities and aggregates TiN inclusions, it mainly harms the material in two aspects: on the one hand, due to the combination of Ti element and N element, C element cannot or only a small amount of combination with Ti, resulting in increased intergranular corrosion tendency, while TiN inclusions themselves also reduce the corrosion resistance of the material; On the other hand, TiN inclusions are brittle phases with high hardness, angular shape and easy to aggregate, it destroys the continuity of steel and causes stress concentration, reduces the plasticity, toughness and fatigue resistance of steel, and is easy to become a crack source under the action of external force.
 

3. Conclusion

This batch of 06 Cr19Ni10(SUS304) stainless steel materials have more δ ferrite in the material due to unreasonable alloy element ratio, low nickel content and nickel equivalent, and high residual strong ferrite forming element Mo, which is the main cause of surface cold heading cracks. TiN inclusions in the material, which are aggregated and distributed in chain and network, not only have superimposed effects on cold heading cracks, and it will affect its corrosion resistance.

Source: Golden Spider Fastener Web