Investigation of the influence volume-surface hardening (VSH) on the tendency to corrosion of 20GFL steel used for cast parts of the railway transport
Cast parts of railway transport made of 20GFL steel are subjected to heavy loads due to working in harsh climatic conditions. So, an actual issue is to improve the operational reliability of critical parts of rolling stock of the railway transport by providing high mechanical properties of the steel used for producing it. A promising method of steel hardening is a method called volume-surface hardening (VSH). VSH includes heating parts in the shaft furnaces, holding and cooling in the fast-moving stream of water which allows to carry out the hardening of the surface layer by producing martensite and also the hardening of the core section of the working parts at once. However, VSH influence on corrosion behavior of steels has not been investigated yet. In this regard, the purpose of the work is the study of the structure, mechanical properties and corrosion resistance of steel 20GFL after normalization and VSH; development of methods of corrosion testing of the samples of the side frame rail transport after the volume-surface hardening
Tasks: 1. Qualitative and quantitative study of the microstructure of the 20GFL steel in the initial state (after normalization) and after the VSH 2. Measurement of the microhardness over the cross section samples 3. Development and approbation of the methods testing corrosion resistance 4. Comparison of tne tendency to corrosion of steel 20GFL in normalized condition and after the VSH.
Metallographic analysis showed that:
- Lateral frames fragments microstructure after normalization is a ferrite-perlite mixture with an average grain size of 20 ± 10 microns;
- VSH leads to the changes in the structure of the fragment thtouth the cross-section: there is a decarbonized layer of 100 ± 30 mm and then hardened zone consisting of martensite and troostite and then troostite, thin plate ferrite-cementite mixture of the troostite, decarbonized layer (in the direction from the outer edge of the sample to inner, respectively).
Changes in microhardness after VSH showed a gradient nature of the cross section of the frames, and micro-hardness measurement results are consistent with the results of microstructural analysis: a) after the normalization of the mean value of 150-200 HV; b) after the VSH - 200-500 HV, the maximum microhardness is in the hardened zone - 500 HV, corresponds to the minimum and decarbonized layer is 200 HV
Based on the results of metallographic analysis and microhardness analysis and analytical review of the literature on the corrosion of low alloyed and carbon steel were developed and tested the methods of corrosion of 20GFL steel in the atmosphere and salt spray samples cut from the side of the frame. The preliminary results showed no substantial corrosion resistance, depending on the type of heat treatment. In testing the samples in salt spray corrosion set different speed after normalization and AEs different structural domains: decarbonized layer corrodes faster than 2.9 times than the hardened layer with a structure troostomartensita.