Heat treatment and strength test of welded joints
Because 65Mn steel has a tendency to overheat, the welding heat affected zone has a great influence on the mechanical properties of the joint. The 65Mn steel wire with a diameter of 0.7 mm is very hard and brittle after butt-welding by argon arc welding. When the welding point is slightly bent, it will be brittle fracture at the fusion line or weld, and the fracture will show obvious brittle fracture morphology. The resulting joint consists of a weld and a heat-affected zone, and the microhardness of each area from the center of the weld to the base metal is tested along the axis of the joint. The measurement results show that from the base material to the heat-affected zone and the middle of the weld, the microhardness increases sharply, and the hardness of the middle of the weld reaches HV 1 060, which indicates that the heat-affected zone and the middle of the weld have formed hard and brittle structures. For this kind of joint with hard and brittle structure, in order to improve its toughness and plasticity, reduce its hardness, and obtain an appropriate match of hardness, strength, plasticity and toughness, the welded joint must be properly tempered. After heat treatment, the brittleness of the heat-affected zone should be eliminated, and the base material should be able to maintain a certain strength and elasticity. The tempering is carried out in a box-type resistance furnace, and the tempering process is shown in Table 1. Carefully polish the welded joints of the steel wires after tempering to make their diameters approximately equal to those of the base material, and then conduct a tensile test on the WE-50 tensile testing machine. Take three samples of each tempering treatment and take the average value of their tensile forces.
It can be seen from the test that after heat treatment above 330℃, the elasticity of the base material basically disappears, and the fracture occurs at the base material, but not in the solder joint and its heat-affected zone. This shows that the brittleness of the heat-affected zone is completely Disappeared, but the strength of the base material was greatly shaved off (after tests, the tensile strength of the base material used was 1 663 MPa). When the temperature is kept at 260°C for 10 min, although the elasticity of the material remains basically unchanged, the brittleness of the heat affected zone cannot be eliminated. The effect is best when the heating temperature is 280℃ and the heat preservation time is 10 min. The tensile strength of the heat-affected zone is only about 20% lower than that of the base material, while the elasticity of the base material disappears less. The 280°C tempered welding head was tested for the microhardness of each zone on the longitudinal section along the axial direction. It was found that the highest hardness value at the weld was reduced to about HV 500, which was about 1 times lower than the untreated hardness.
The welded ring steel wire should not only meet certain strength and elasticity requirements, but also have a certain fatigue strength.
(1) Non-melting polar argon gas shielded welding can be used to weld φ0.7 mm 65Mn steel wire to obtain a good-looking welded joint, and no filler metal can be added during welding. The welding current intensity is more suitable for 10 A. If the current is too high, it will cause spatter and collapse. If the current is too small, it is difficult to start the arc and the arc is unstable.
(2) The welded joint is very hard and brittle, so proper heat treatment must be carried out. The use of a tempering process with a heating temperature of 280°C and a heat preservation time of 10 min for the welded joint can make the joint’s tensile strength up to 1 370 MPa and higher fatigue strength.
Improvement of heat treatment process for 65Mn steel spring support
Improve the heat treatment process for spring support, increase process holes, use new quenching hangers, and adopt surface sandblasting treatment to solve the problems of uneven hardness, shape changes and unclean surfaces after heat treatment.
The 65Mn steel spring support is the spring used for the left and right side support linings of the light passenger car body of our company, and the hardness after quenching is 45-50HRC. In response to the uneven hardness and shape distortion caused by heat treatment, and the surface is not clean, rust, salt, and not easy to spot welding, we improved the heat treatment process, used a new quenching hanger and the surface was sandblasted. Practice has proved that its effect is remarkable.
Quenching Process of 65Mn Steel Spring Support
When the 65Mn steel spring support is quenched in a medium-temperature salt bath furnace, the quenching temperature is 810～830℃. After heat treatment, it is found that the quenching hardness is uneven and the difference in height is very different. The hanging utensils used in the salt furnace are made of barbed wire, with pockets, 20 pieces each time. Because there are more point-to-surface contacts between parts. It is easy to cause uneven hardness during quenching, affecting the quality of the workpiece, and low or inelasticity. For this reason, the quenching hanger and the workpiece were improved respectively. At 5mm from the edge of the spring support, add a process hole of 1 mm, and tie each piece with iron wire, leaving a certain gap between the pieces. Each string is 20 pieces, each furnace is equipped with 4 strings, and the furnace capacity is 80 pieces. In this way, the injustice makes the quenching hardness of the workpiece uniform, and the furnace load is increased by 4 times, which improves the work efficiency.