High temperature resistant rods

Detailed explanation of heat treatment process for high temperature resistant bars

In high-temperature industrial fields such as aerospace, energy and electricity, metallurgy and chemical industry, high-temperature resistant rods are the basic materials for manufacturing key components, and their performance directly affects the reliability and service life of the equipment. The heat treatment process is the core means to tap the performance potential of high-temperature resistant rods and optimize the organizational structure. Through different heating, insulation and cooling methods, the key properties of the rods such as high-temperature strength, oxidation resistance, and thermal fatigue resistance can be significantly improved. Next, we will explore in depth the commonly used heat treatment processes for high-temperature resistant rods and their principles and applications.

1. Annealing process: refine the structure and eliminate stress

1. Full annealing

Full annealing is mainly used for high temperature resistant bars made of hypoeutectoid steel, such as low alloy heat resistant steel. The process is to heat the bar to 30 - 50℃ above Ac3 (upper critical point), keep it warm for a certain period of time to make the structure completely austenitized, and then slowly cool it with the furnace. In this process, the coarse grains are refined, the internal residual stress is eliminated, and the hardness of the material is reduced, and the cutting performance is improved. For example, when manufacturing 15CrMo bars for high temperature and high pressure pipelines, full annealing can make the pearlite and ferrite structures evenly distributed, and improve the plasticity and toughness of the material at high temperature.

2. Spheroidizing annealing

Spheroidizing annealing is suitable for high temperature resistant bars made of hypereutectoid steel or alloy tool steel, such as Cr12MoV die steel. The process is to heat the bar to 20 - 40℃ above Ac1 (lower critical point), keep it warm and cool it very slowly. During this process, the lamellar cementite gradually transforms into a spherical shape, which reduces the hardness of the material, improves the toughness, and also improves the process performance during subsequent heat treatment. For mold bars that need to withstand repeated thermal shocks, the spherical structure after spheroidizing annealing can effectively inhibit the generation and expansion of cracks.

3. Stress relief annealing

Stress relief annealing is mainly used to eliminate the residual stress generated in the bar during forging, rolling or welding. The bar is heated to a temperature below Ac1 (usually 500-650℃), kept warm for a period of time and then slowly cooled. This process does not change the organizational structure of the bar, but can improve its dimensional stability and fatigue resistance by releasing internal stress. For example, in the manufacture of high-temperature alloy bars for aircraft engine turbine blades, stress relief annealing can prevent the blades from failing due to stress concentration when rotating at high temperature and high speed.

2. Normalizing process: improve performance and increase strength

The normalizing process is to heat the high temperature resistant bar to 30 - 50℃ above Ac3 or Accm (upper critical point of hypereutectoid steel), keep it warm and then cool it quickly in the air. Compared with annealing, normalizing has a faster cooling rate and can obtain a finer structure, thereby improving the strength and hardness of the bar. For high temperature resistant bars made of medium carbon alloy steel, such as 42CrMo, normalizing can refine the grains and improve its comprehensive mechanical properties, so that it can still maintain a high load-bearing capacity at high temperatures. The normalizing process is often used in the manufacture of parts that have high performance requirements but do not need to reach the quenching and tempering strength level, such as supporting structures for large high temperature equipment.

3. Quenching and tempering process: the key to improving comprehensive performance

1. Quenching

Quenching is a process of heating the high temperature resistant bar to above the critical temperature, keeping it warm for a certain period of time and then cooling it rapidly. Rapid cooling transforms the austenite structure into martensite or bainite, significantly improving the hardness and strength of the material. The quenching process of high temperature resistant bars of different materials is different:

Medium carbon alloy steel: such as 35CrMo, the quenching temperature is generally 850 - 880℃, using oil cooling or water cooling. The hardness of the bar after quenching is greatly improved, but the brittleness also increases, and it needs to be tempered in time.

High temperature alloys: such as GH4169, the quenching temperature is relatively high, up to 950 - 1050 ° C, and air cooling or water quenching is mostly used to obtain fine grains and strengthening phases, thereby improving its high temperature strength and oxidation resistance.

2. Tempering

Tempering is an essential subsequent process after quenching. According to the tempering temperature, it can be divided into low-temperature tempering, medium-temperature tempering and high-temperature tempering:

Low temperature tempering: temperature range 150 - 250℃, mainly used to reduce quenching stress, reduce brittleness and maintain high hardness. Suitable for high temperature resistant parts requiring high hardness and wear resistance, such as bars for high temperature cutting tools.

Medium temperature tempering: The temperature is between 350 and 500°C, which can obtain higher elasticity and yield strength. It is often used to make rods for parts such as springs and elastic elements that work at high temperatures.

High temperature tempering: also known as quenching and tempering treatment, the temperature is 500 - 650℃. After quenching and high temperature tempering, the bar can obtain good comprehensive mechanical properties, with both high strength and good toughness. For example, the 25Cr2MoV bar used for turbine rotors can operate stably for a long time in a high temperature and high pressure steam environment after quenching and tempering treatment.

4. Solution treatment and aging treatment: optimizing high temperature alloy properties

1. Solution treatment

Solution treatment is mainly used for high temperature resistant bars made of high temperature alloys, such as Inconel 718. The bars are heated to the single-phase austenite region, kept warm for a certain period of time to allow the alloy elements to fully dissolve in the austenite, and then cooled rapidly. This process can obtain a uniform supersaturated solid solution, improve the plasticity and toughness of the alloy, and prepare the organization for subsequent aging treatment. For example, high temperature alloy bars used in the combustion chamber of an aircraft engine can better withstand the erosion and thermal stress of high temperature gas after solution treatment.

2. Timeliness treatment

Aging treatment is divided into natural aging and artificial aging. Artificial aging is to heat the bar after solution treatment to a certain temperature (lower than the solution temperature), keep it warm for a certain period of time and then cool it. During the aging process, the supersaturated solid solution decomposes and precipitates fine and dispersed strengthening phases, thereby significantly improving the strength, hardness and creep resistance of the alloy. For parts that bear loads for a long time at high temperatures, such as bars for gas turbine blades, aging treatment can effectively improve their high-temperature endurance strength and extend their service life.

5. Surface heat treatment process: enhancing surface performance

1. Carburizing treatment

Carburizing is suitable for high temperature resistant bars made of low carbon alloy steel. It can improve the surface hardness and wear resistance while maintaining the toughness of the core. The bar is placed in a carburizing medium and heated to 900 - 950 ° C to allow carbon atoms to penetrate into the surface layer, followed by quenching and low temperature tempering. For example, when manufacturing bars for gears working at high temperatures, carburizing can form a high-hardness carburized layer on the surface to resist wear and fatigue, while the core still maintains good toughness to prevent fracture.

2. Nitriding treatment

Nitriding treatment uses nitrogen atoms to penetrate the surface of the rod to form a nitride layer, which improves the surface hardness, wear resistance, corrosion resistance and thermal fatigue resistance. Compared with carburizing, the nitriding treatment temperature is lower (generally 500-650℃), the deformation is small, and the nitriding layer has good red hardness, that is, it can still maintain high hardness at high temperatures. For example, when manufacturing stainless steel bars for high-temperature and high-pressure valves, nitriding treatment can significantly improve the wear resistance and corrosion resistance of the valve sealing surface and extend the service life of the valve.

There are many types of heat treatment processes for high temperature resistant bars, and each process has its unique role and scope of application. In actual production, it is necessary to reasonably select and combine heat treatment processes according to the material, use requirements and performance goals of the bars to give full play to the performance advantages of high temperature resistant bars and meet the increasingly stringent needs of the high temperature industry.