At present, the common materials of parts in the domestic hydraulic support industry mainly include 27SiMn steel, 30CrMo steel, 42CrMo steel and ZG30Cr06A. In recent years, 27CrNiMo steel has also been used in the production of hydraulic supports because it has added higher content of Ni to improve the toughness of steel. Because the connection between the mining support parts and the main structural parts of the hydraulic support is welding, and the hydraulic support is of high quality, so it can only be slowly cooled and cannot be tempered after welding, so the selection of materials requires not only the strength index to meet the mechanical design requirements, but also the weldability of the final selected materials.
(1) Chemical composition of the material The main chemical composition of several mining support parts is shown in Table 1. Its carbon equivalent is calculated according to the calculation formula [CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15] and the chemical composition of the corresponding national standard for each material.
| Material Science | C | Si | Mn | Cr | Mo | Ni | Carbon equivalent |
| 27SiMn | 0.24～0.32 | 1.10～1.40 | 1.10～1.40 | — | — | — | 0.42～0.68 |
| 27CrNiMo | 0.24～0.30 | 0.17～0.37 | 0.60～0.95 | 0.35～0.65 | 0.35～0.75 | 0.15～0.25 | 0.49～0.76 |
| 30CrMo | 0.26～0.33 | 0.17～0.37 | 0.40～0.70 | 0.80～1.10 | 0.15～0.25 | — | 0.52～0.72 |
| 2G30Cr06A | 0.24～0.32 | 0.20～0.40 | 0.50～0.80 | 0.50～0.80 | — | — | 0.42～0.61 |
| 42CrMo | 0.38～0.45 | 0.17～0.37 | 0.50～0.80 | 0.90～1.20 | 0.15～0.25 | — | 0.67～0.87 |
(2) The weldability of the material was tested according to the national standard GB/T 4675.1-1984 “Weldability Test Method for Welding Cracks in Oblique Y-Groove”. The 27SiMn steel and 30CrMo steel were tested at the preheating temperature of 100 ℃, and the average section crack rate was 25.87% and 28.44%, respectively. This indicates that the weldability of the two materials is poor. If they are used in actual production, even if they are welded with the low carbon steel with good weldability, the higher temperature preheating should be used, Generally, 150~200 ℃ preheating is used in production to avoid cold cracks and affect weld quality.
(3) The mechanical properties of materials are tested by casting or forging for some alternative materials. After quenching and tempering, the mechanical properties are shown in Table 2.
| Material Science | Tensile strength/MPa | Yield strength/MPa | Elongation after fracture (%) | Impact absorbed energy/J | Marks |
| 27SiMn | 870 | 750 | 13 | 31 | Casting+tempering |
| 27CrNiMo | 910 | 764 | 14 | 63 | Casting+tempering |
| ZG30Cr06A | 790 | 675 | 8 | 46 | Casting+tempering |
| 42CrMo | 1080 | 945 | 12.5 | 51 | Casting+tempering |
The strength of the above materials meets the mechanical design requirements, but because 27CrNiMo steel has better toughness and higher elongation after fracture, it is more suitable for the production of mining support parts that are in service under severe conditions.
