First, methods for preheating the deformation of straight seam steel pipes.
1. Reasonable material selection. For precision complex molds, high-quality micro-deformation mold steel should be selected. For mold steel with severe carbide segregation, reasonable casting and tempering heat treatment should be performed. For larger mold steels or those that cannot be cast, solution double-refining heat treatment can be performed. Reasonable selection of heating temperature and control of heating rate are crucial. For precision complex molds, slow heating, preheating, and other balanced heating methods can be used to reduce heat treatment deformation.
2. Correct heat treatment process operation and reasonable tempering heat treatment process are also effective methods to reduce deformation of precision complex molds. The causes of deformation of precision complex molds are often complex, but as long as the deformation rules are understood, the causes are analyzed, and appropriate methods are used to prevent deformation, it can be reduced and controlled.
3. Precision complex molds should undergo preheat treatment to eliminate residual stress generated during machining. For precision and complex molds, vacuum heating quenching and deep cryogenic treatment after quenching should be used whenever possible, provided conditions permit. Pre-cooling, graded cooling quenching, or warm quenching processes should be used whenever possible, while ensuring mold hardness.
4. Mold design should be reasonable, with minimal thickness variations and symmetrical shapes. For molds with significant deformation, deformation patterns should be understood, and machining allowances should be reserved. For large, precision, and complex molds, a combination design can be used. For some precision and complex molds, pre-heat treatment, aging heat treatment, and tempering and nitriding heat treatment can be used to control mold precision. When repairing defects such as sand holes, porosity, and wear in molds, equipment with minimal heat impact, such as cold welding machines, should be used to avoid deformation during the repair process.
Second, the annealing process for straight seam steel pipes.
Annealing of straight seam steel pipes involves heating the steel pipe to a predetermined temperature and holding it at that temperature, then slowly cooling it to room temperature. There are several types of annealing, including annealing, spheroidizing annealing, and stress-relief annealing.
1. Heating the steel pipe to a predetermined temperature, holding it for a period of time, and then slowly cooling it in the furnace is called annealing. The purpose is to reduce the hardness of the steel and eliminate uneven microstructure and internal stress.
2. Spheroidizing annealing involves heating the steel pipe to 750 degrees Celsius, holding it at that temperature for a period of time, slowly cooling it to below 500 degrees Celsius, and then cooling it in air. Its purpose is to reduce the hardness and improve the machinability of the steel, and it is mainly used for high-carbon steel.
3. Stress-relieving annealing, also known as low-temperature annealing, involves heating the steel to 500-600 degrees Celsius, holding it at that temperature for a period of time, slowly cooling it in the furnace to below 300 degrees Celsius, and then cooling it to room temperature. The microstructure does not change during annealing; its main function is to eliminate internal stress in the metal.
4. Normalizing is a heat treatment process in which the steel pipe is heated to 30-50 degrees Celsius above its critical temperature, held at that temperature for an appropriate time, and then cooled in still air. The main purpose of normalizing is to refine the microstructure, improve the properties of the steel, and obtain a near-equilibrium microstructure. Compared to annealing, the main difference is that normalizing has a slightly faster cooling rate, resulting in a shorter production cycle. Therefore, when annealing and normalizing can both achieve the performance requirements of the parts, normalizing should be chosen whenever possible.
5. Quenching: The heat treatment process of heating steel pipes to a temperature above the critical point (840-860℃ for 45 steel, 760-780℃ for carbon tool steel), holding it for a specified time, and then cooling it at an appropriate rate in water (oil) to obtain a martensitic or bainitic structure is called quenching. The main difference between quenching and annealing/normalizing is the faster cooling rate, the purpose of which is to obtain a martensitic structure. Martensitic structure is an unbalanced structure obtained after quenching; it has high hardness but poor plasticity and toughness. The hardness of martensite increases with the carbon content of the steel.
6. Tempering: The heat treatment process of hardening steel pipes, then reheating them to a temperature below the critical temperature, holding them for a specified time, and then cooling them to room temperature is called tempering. Quenched steel parts generally cannot be used directly and must be tempered before use. Because quenched steel has high hardness and brittleness, direct use often results in brittle fracture. Tempering can eliminate or reduce internal stress, reduce brittleness, and improve toughness; on the other hand, it can adjust the mechanical properties of quenched steel to achieve the steel’s performance requirements. Depending on the tempering temperature, tempering can be divided into three types: low-temperature tempering, medium-temperature tempering, and high-temperature tempering.
1) Low-temperature tempering: 150–250°C; reduces internal stress and brittleness, while maintaining high hardness and wear resistance after quenching.
2) Medium-temperature tempering: 350–500°C; improves elasticity and strength.
3) High-temperature tempering: 500–650°C; tempering quenched steel parts at temperatures above 500°C is called high-temperature tempering. After high-temperature quenching, quenched steel parts possess comprehensive mechanical properties (both strength and hardness, as well as plasticity and toughness). Therefore, medium carbon steel and medium carbon alloy steel often undergo high-temperature tempering after quenching. This is commonly used for shaft parts. Quenching + high-temperature tempering is called tempering treatment.
Post time: Dec-04-2025