When must stress relief and stress relief tempering be done for LPDC molds?
H13 molds will generate residual stress after processing, welding, and nitriding. If the stress is not released, the risk of deformation and cracking in the later stage will increase significantly, which is the root cause of many hidden failures.
1、 Source, magnitude, and corresponding tempering process of residual stress
After rough machining, stress relief tempering is carried out at a temperature of 550-580 ℃ for 2-4 hours, followed by slow cooling in the furnace to release milling and turning stresses; Rough machining does not relieve stress, and after precision machining, it slowly deforms and drifts in size by 0.03-0.06mm after being left for 7-15 days.
After welding repair, the stress relief tempering is carried out at a temperature of 520-560 ℃, and the insulation time is determined by the thickness of the welding. The insulation time is 1 hour for every 25mm thickness to eliminate welding thermal stress and prevent cracking in the welding area.
Stress must be relieved before nitriding. If there is residual stress during precision machining, the stress will be released during the nitriding heating process, causing the contour of the mold cavity to deviate and the assembly size to exceed the tolerance.
The matrix stress after quenching and tempering of the mold is eliminated by factory heat treatment; The secondary stress relief should not exceed the tempering temperature of the substrate, otherwise the hardness of the substrate will decrease.
Xinfeng Machinery has included the stress relief process in the process card, and mandatory verification is required for the first three nodes of rough machining, welding, and nitriding.
2、 Stress relief selection criteria under different working conditions
Mass production molds require a precision machining dimensional tolerance of ± 0.05mm, and after rough machining, stress must be relieved to ensure long-term dimensional stability.
Trial molds and parts with loose tolerances can be omitted when the budget is tight, but customers should be informed of the risk of deformation in the later stage.
The mold modified by welding, regardless of size, must be stress relieved after welding, and stress concentration in the welding area is a high risk factor for thermal cracking.
Precision moving parts such as sliders and inserts must be stress relieved after processing to prevent deformation, jamming, and changes in fit clearance in the later stages.
3、 Differences in the audit of stress relief processes by overseas customers
Many supporting factories in Southeast Asia omit independent stress relief processes and rely on subsequent mold testing to naturally release, only accepting castings with loose tolerances.
The heat treatment process documents for North American automotive molds must include records of stress relief and tempering, as well as traceable insulation curves and heating and cooling rates, which are PPAP audit items.
European customers value the long-term service life of molds, and even if the cost increases, they require two rounds of stress relief after rough machining and welding to reduce the risk of premature mold scrapping.
Middle Eastern construction machinery molds have loose tolerances, but due to the large tonnage of the molds, production losses are high once they crack. Most customers agree to add stress relief processes.
4、 Hidden faults caused by residual stress
After precision machining, the size is qualified, but after a period of production, it slowly deforms, the thickness of the cavity wall and the size of the reverse buckle drift, and the batch size exceeds the tolerance.
After nitriding, the cavity deforms, the parting surface is poorly adhered, and there are irregular burrs. Repeatedly grinding the parting surface makes it thinner and thinner.
The welding area did not relieve stress, and under cold and hot shock, cracks rapidly propagated in the welding area, extending into the mold cavity and causing linear defects in the casting.
5、 Long tail traffic layout ideas
Stress relief tempering, residual stress, and mold deformation belong to the search category of heat treatment matching, targeting heat treatment engineers and mold engineers.
Keywords: LPDC mold stress relief tempering, H13 mold residual stress after welding;
Question class: When to do stress relief for LPDC die;
Tutorial class: stress relief temperature for H13 casting mold.
Suitable for heat treatment technology topics, undertaking mold heat treatment and welding matching flow.
6、 Common misconceptions in the industry
Misconception 1: After quenching and tempering, there is no stress. Machining, milling, and welding will introduce new residual stresses, which are not the same as the original quenching stress.
Misconception 2: The higher the stress relief temperature, the better. If it exceeds the tempering temperature of the substrate, the hardness of the mold substrate will decrease, and the wear resistance and heat fatigue resistance will weaken.
Misconception 3: Small molds do not require stress reduction, precision small parts have high tolerance requirements, and even slight stress deformation can cause assembly scrap.
FAQ
Q: What is the recommended temperature for stress relief after rough machining of H13?
A: 550-580 ℃, keep warm for 2-4 hours, and cool slowly with the furnace.
Q: Why is it necessary to relieve stress after welding?
A: Release welding thermal stress to avoid thermal cracks in the welding area in the later stage.
Q: What is the purpose of pre nitriding stress?
A: Prevent the release of nitriding temperature rise stress and the displacement of cavity size.
Q: How long does it take for deformation to appear without stress relief?
A: After precision machining, let it stand for 7-15 days, and some molds begin to slowly deform.
Q: What are the stress audit requirements for North American customers?
A: Provide complete tempering curve records and include them in PPAP data.
Q: Can the stress relief temperature be higher than the tempering temperature of the substrate?
A: No, it will cause a decrease in the hardness of the substrate.
Q: Can stress relief be omitted for trial molds?
A: Loose tolerance can save costs, but it is necessary to inform the deformation risk in advance.
Q: Why do slider inserts need to relieve stress?
A: Prevent deformation in the later stage, change the fit clearance, and prevent jamming and biting.
