51CrV4 Spring Steel Bar


51CrV4 (DIN 1.8159 or JIS SUP10 ) is a high-end application spring steel. This steel has good mechanical properties and process properties, and high hardenability. Add vanadium to refine the steel's grains, reduce overheating sensitivity, and improve strength and toughness. Let this steel has high fatigue strength, high yield ratio, but poor weldability, low plasticity due to cold deformation. It is often used to make large cross-section, high-load important springs, and valve springs, piston springs, and safety valve springs with an operating temperature of <300 ℃.


Chemical composition:

 0.47~0.55≤0.40 0.70~1.10 ≤0.030 ≤0.030  0.90~1.20 ≤0.35 ≤0.25 0.10~0.25



Mechanical Properties:


Yield strength σs (MPa):Tensile strength σb (MPa): Elongation δ5 (%): Reduction of area ψ (%): Hardness: 
≥1127(115) ≥1274(130) ≥10  ≥40 Hot rolled:≤321HB
Cold drawn + heat treatment:≤321HB


Heat treatment specification: 

Quenching 850℃±20℃, oil cooling; tempering 500℃±50℃ (for special needs, ±30℃).


Delivery status: 

Hot-rolled steel is delivered in heat-treated or non-heat-treated state, and cold-drawn steel is delivered in heat-treated state.


Welding performance:


Argon arc welding butt welding process

In order to reduce the consumption of the electrode, the DC positive connection is selected for the butt welding test of the wire, that is, the DC power supply is selected, the wire is connected to the positive electrode of the power source, and the tungsten electrode is connected to the negative electrode of the power source. The tungsten electrode containing 1% or 2% thorium oxide has high electron emission efficiency, good current carrying capacity, good anti-pollution performance, easy arc ignition and relatively stable arc. For ease of operation, a finer thorium tungsten electrode with a diameter of 2 mm was selected, and the tip of the electrode was sharpened. Since the lower arc voltage characteristic of argon is particularly beneficial for manual arc welding of thin plates and wires, argon is chosen as the shielding gas.


The test uses a DC manual argon arc welding machine. Before welding, both ends of the steel wire are carefully smoothed. In order to prevent pores in the solder joints, the oil stains on the ends are cleaned with acetone. Place the flat wire on both ends on a flat and clean alignment plate, align the two ends without leaving a gap at the joint, and press the two sides of the joint with a weight. Connect the wire to the positive pole of the welding machine and the tungsten pole to the negative pole, and adjust the current to 20 A, 15 A, 10 A, and 8 A for welding. When welding, ignite the ignition arc next to the joint and make it stable. Move the arc to the joint to melt the joint metal and quickly extinguish the arc. At the same time, slightly apply upsetting force. After cooling, the welding process will be completed. Use filler wire.


The test found that when the welding current is 20 A, the arc burns violently, the metal spatter at the joint is serious, and the welding joint collapses seriously. When the current is adjusted to 15 A, the arc burns more smoothly and there is less splashing in the molten pool, but the weld still collapses. But when the current drops to 10 A, the arc is easy to start, the arc burns stably, and there is no collapse in the weld. Figure 2 shows the shape of the welded joint taken with a digital camera under a Leica MZ6 stereo microscope when the welding current is 10 A. It can be seen that the cylindricity of the joint is good, and it can meet the requirements of a wire saw after it is polished. When the current is adjusted below 8 A, it is difficult to start the arc and the arc is unstable, making it difficult to complete the welding process.


Because 51crv4 steel has a tendency to overheat, the heat-affected zone of welding 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 is composed 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 shows 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 at the same time, 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 metal, 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. When the heating temperature is 280℃ and the heat preservation time is 10 min, the effect is best. 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℃ tempered welding head was tested along the axial direction of the microhardness of each zone on the longitudinal section. 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.


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