Material selection and heat treatment process of shaft parts

In this paper, the material selection and corresponding heat treatment process of various shaft parts are introduced according to the working conditions and failure modes of shaft parts.

1. Working conditions of shafts

Machine tool spindles, spline shafts, screws, and internal combustion engine crankshafts, connecting rods and, automobile driveshafts, half shafts belong to the shaft parts; they are one of the important basic parts in the machinery industry, all the rotary movement of the parts are mounted on the shaft. Most of the working conditions of the shaft:

• (1) The transmission of torque, subject to alternating torsional loads, often also subject to alternating bending stresses;
• (2) The journal to withstand large friction;
• (3) Most of the overload or impact load to withstand a certain degree.

2. Shaft failure form

According to the working characteristics, the main failure forms of shaft parts are as follows:

• (1) Fatigue fracture. The long-term action of alternating loads causes fatigue fracture, mainly torsional and bending fatigue. This is the most important form of failure.
• (2) Fracture failure. Due to overload or impact load, resulting in shaft fracture or twisted.
• (3) Wear failure. Excessive wear of the journal or spline changes the shape and size of the shaft.

3. Performance requirements for shaft materials

According to the shaft working conditions and failure forms, the shaft materials put forward the following requirements:

• (1) High fatigue strength to prevent fatigue fracture;
• (2) Good overall mechanical properties to reduce the stress concentration effect and notch sensitivity to prevent the impact or overload fracture, the need for shaft strength and plasticity, toughness and have a good match;
• (3) Good abrasion resistance to prevent wear and tear of the journal.

4. Shaft parts of commonly used materials

To consider the strength and toughness while taking into account the fatigue resistance, shaft parts generally often choose the following three materials.

4.1 Selection of tempered steel.

According to the specific working conditions, tempered steel can be divided into three cases. Smaller forces, precision requirements are not high, and the impact load is not the large shaft; for example, C620 and CW61100, such as the spindle material, generally choose carbon steel (such as 45# steel); and at the same time, bear the axial and torsion alternating loads, precision requirements are high, but also to withstand a certain degree of impact of the more important shaft, for example, C6132 lathe spindle, M7475B grinding wheel spindle, etc., the selection of materials, you can choose to use the alloy tempering Steel (such as 40Cr, 40MnB); to withstand higher alternating loads, impact loads, by strong friction, high speed, high precision requirements of the important axis, such as T4240A coordinate boring machine spindle, C21506 multi-axis automatic lathe center shaft, T68 boring bar, and other parts, nitriding and tempered steel can be selected (such as 38CrMoAl), such materials should be tempered and then nitriding treatment to meet the Performance requirements.

4.2 Carburized steel selection.

To withstand heavy alternating loads, shock loads, and strong friction of the shaft, for example, Y236 gear planer, Y58 gear shaper spindle, SG8030 precision lathe spindle, you can choose alloy carburized steel (such as 20Cr, 20CrMnTi), this type of steel carburized, quenched, tempered to ensure that the use of performance.

4.3 Alloy tool steel or rolling bearing steel selection.

To withstand medium or heavy load, impact load is small, high precision requirements, the journal part of a higher wear resistance requirements of the shaft, such as MQ1420 \MB1432A grinding wheel spindle, can choose alloy tool steel or rolling bearing steel (such as 9Mn2V, Gr15). These shafts, after tempering, journal and square head at the local quenching to ensure the use of performance.
Mainly subjected to alternating torsional loads, the impact is a small, wear-resistant, complex shaft structure, such as an internal combustion engine crankshaft, which can be selected as ductile cast iron. Large low-speed shafts can be used cast steel.

5. Typical shaft parts with materials and heat treatment process

5.1 Tempered steel produced by the shaft

General process route: material → forging → normalizing → rough machining → tempering → finishing → high-frequency quenching and low-temperature tempering of the journal → grinding.

For example, 45# steel is used for the spindle of the C620 lathe. Normalizing: 840-860 ℃, air cooling, ≤ 229HB. tempering: 820-840 ℃ water quenching, 550-580 ℃ tempering, 220-250HB. journal surface quenching: 860-900 ℃ (water quenching), 160-250 ℃ tempering, 46-54HRC.
H155B cylindrical grinding machine grinding wheel holder spindle, the material is 38CrMoAl. tempering requirements for hardness 250-280HB, nitriding layer surface hardness ≥ 900HV, nitriding layer depth 0.4-0.55mm.

• Annealing: 840-870 ℃ insulation 5h, furnace cooling to 550 ℃ out of the air-cooled.
• Tempering: 930-950 ℃ insulation 3h, quenching oil cold through; high-temperature tempering 620-650 ℃ insulation 5h after air-cooled.
• De-stressing: furnace temperature ≤ 350 °C, heating speed ≤ 50 °C/h, 620-650 °C insulation 10-12h, furnace cooling to 350 °C out of the air-cooled, cooling speed ≤ 30 °C/h.
• Nitriding: 500-510 ℃ insulation 25h (ammonia decomposition rate of 18% -25%), 510-520 ℃ insulation 25h (ammonia decomposition rate of 40% -50%), denitrification 2h (ammonia decomposition rate of ≥ 80%), furnace cooling to ≤ 150 ℃, out of the air-cooled. When the brittleness of the nitrided layer is greater than grade 3, the brittleness elimination treatment must be carried out by holding it at 400°C for 6h.

5.2 Carburized steel produced by the shaft

The general process route: material → forging → normalizing → rough machining → carburizing → quenching and low temperature tempering → grinding.

For example, the Y7163 gear is grinding machine spindle selection of carburized steel: 20CrMnTi.

• Normalizing: 950-970 ℃, air cooling. Carburizing, quenching: 910-940 ℃ carburizing, 320-340 ℃ oil quenching, 160-200 ℃ tempering.
• Surface hardness: ≥59HRC.

M1350 universal cylindrical grinder headstock spindle, the material is 20Cr, the carburization depth of 0.9-1.4mm, quenched surface hardness of 59-64HRC, radial runout ≤ 0.25mm.

• Gas carburizing: 930 ℃ insulation 6.5h carburizing, furnace cooling to 650 ℃ out of the air-cooled.
• Salt bath quenching: 790 ℃ insulation 25min, quenching brine 8s transferred to oil cooling. Tempering: 160 ℃ insulation 8h after air-cooled.

5.3 Tool steel or rolling bearing steel produced by the shaft

The general process route: material → forging → spheroidal annealing → rough machining → tempering → finishing machining → surface quenching and low-temperature tempering → grinding.

Such as the MB1432A grinding wheel spindle selection of 9Mn2V steel.

• Spheroidal annealing: 760 ℃, furnace cooling to 500 ℃ below the air-cooled, ≤ 195 HB. tempering: 800 ℃, oil quenching, 650-700 ℃ tempering.
• Hardness: 22-26 HRC. Local quenching: 840-860 ℃ oil quenching, 160-200 ℃ tempering.
• Surface hardness: ≥50HRC.

Author: Yu Jingcheng, Luo Dejun, Wang Jing