Classification of heat treatment process
What is the role of heat treatment?
Table of Contents
The role of heat treatment is to improve the mechanical properties of the material, eliminate residual stresses and improve the machinability of the metal material. According to the different purposes of heat treatment, heat treatment processes can be divided into two categories: preparatory heat treatment and final heat treatment.
Preliminary heat treatment
The purpose of the preparatory heat treatment is to improve the processing properties, eliminate internal stresses and prepare good metallurgical organization for the final heat treatment. Its heat treatment processes are annealing, normalizing, aging, tempering, etc.
1）Annealing and normalizing
Annealing and normalizing are used for the rough billet after thermal processing. Carbon content greater than 0.5% of carbon steel and alloy steel, in order to reduce its hardness easy to cut, often using annealing treatment; carbon content of less than 0.5% of carbon steel and alloy steel, in order to avoid its hardness is too low cutting sticky knife, and the use of normalizing treatment. Annealing and normalizing can still refine the grain, uniform organization, for the future preparation of heat treatment. Annealing and normalizing are often arranged after the manufacture of the blank, before rough machining.
(2) Aging treatment
Ageing treatment is mainly used to eliminate the internal stress generated in the manufacture and machining of the blank.
In order to avoid excessive transport workload, for general accuracy of the parts, in the finishing process before arranging an aging treatment can be. But the higher precision requirements of the parts (such as coordinate boring machine box, etc.), should be arranged for two or several aging treatment process. Simple parts generally can not be aging treatment.
In addition to castings, for some rigid precision parts (such as precision screw), in order to eliminate the processing of internal stress, stable parts processing accuracy, often in rough machining, semi-finishing machining between the arrangement of multiple aging treatment. Some shaft parts processing, straightening process after also to arrange aging treatment.
Tempering that is, after quenching high-temperature tempering treatment, it can obtain a uniform and detailed tempering soxhlet organization, for later surface quenching and nitriding treatment to reduce deformation in preparation, so tempering can also be used as a preparatory heat treatment.
Due to the comprehensive mechanical properties of the parts after tempering, the hardness and wear resistance of some parts do not require high, can also be used as the final heat treatment process.
Final heat treatment
The purpose of the final heat treatment is to improve the hardness, wear resistance and strength and other mechanical properties.
Quenching has surface quenching and overall quenching. Among them, surface hardening because of deformation, oxidation and decarburization is smaller and more widely used, and surface hardening also has the advantages of high external strength, good wear resistance, while maintaining good internal toughness, impact resistance. In order to improve the mechanical properties of surface hardened parts, often need to be tempered or normalized heat treatment as a preparatory heat treatment. The general process route is: the material – forging – normalizing (annealing) – rough machining – tempering – semi-finishing – surface quenching – finishing.
Carburizing quenching for low carbon steel and low alloy steel, first improve the carbon content of the surface layer of the part, after quenching so that the surface layer to obtain a high hardness, while the heart still maintain a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and partial carburizing. Partial carburization of the non-carburizing part to take anti-carburizing measures (copper plating or plating anti-carburizing materials). Due to carburizing quenching deformation, and carburizing depth is generally between 0.5-2mm, so the carburizing process is generally arranged between semi-finishing and finishing.
The process route is generally: the material – forging – normalizing – roughing, semi-finishing – carburizing quenching – finishing. When the partial carburizing parts of the non-carburizing part of the process plan to increase the margin, remove the excess carburizing layer, the process of removing the excess carburizing layer should be arranged after carburizing, quenching before.
Nitriding is to make nitrogen atoms penetrate into the metal surface to obtain a layer of nitrogen compounds treatment. Nitriding layer can improve the surface hardness, wear resistance, fatigue strength and corrosion resistance of the parts. Due to the lower nitriding treatment temperature, small deformation, and nitriding layer is thin (generally not more than 0.6-0.7mm), nitriding process should be arranged as far back as possible, in order to reduce the deformation of nitriding, in the cutting after the general need for stress relief of high-temperature tempering.
For heat treatment of aluminum, the most reported problems include:
- Improper part placement: This can cause part distortion, in large part because the quenchant is not able to transfer heat fast enough to obtain the desired mechanical properties. Improper placement can also cause thermal distortion (because the creep strength of aluminum is not high enough). Proper placement can avoid these problems.
- Excessive heating/heating: This causes thermal distortion and should be prevented. Correct placement of the part helps to heat it up evenly.
- Higher than expected residual stress levels: Heat treatment not only changes mechanical properties, but also directly affects residual stress levels. The following are some possible causes: large differences in cooling rates between the surface and interior during quenching (including when the casting is cooled after solidification); inappropriate ramping rates; temperature changes in intermediate steps; etc. Residual stresses are associated with (large) differences in cooling rates, the cross-sectional thickness of the part, sudden changes in cross-sectional dimensions and the strength of the material. It is important to remember that the stresses caused by quenching are much greater than those caused by other processes (including casting).
- Fluctuations in time/temperature/quench parameters: They will lead to deviations in mechanical and/or physical properties from part to part and from batch to batch. Causes include too long part transfer times, improper quenching (too slow), overheating, underheating, or changes in time-temperature parameters during precipitation hardening. For example, larger particles (precipitates) can precipitate when the time is too long and the temperature is too high.
- Overheating: This tends to produce primary melting or eutectic melting (Figure 2). For example, the solution heat treatment temperature is close to the melting point of many aluminum alloys (especially the 2xxx series, which is often only a few degrees below their melting point). In order to promote the dissolution of solid alloying elements, the proper temperature is required.
- Underheating: This can result in loss of mechanical properties due to insufficient supersaturation. If the aging temperature is too low and/or the aging time is too short, the solute atom concentration zone (GP zone) does not form easily, resulting in low strength after aging.
- Inadequate quenching causes distortion: The problem/difficulty in this area is the movement of the part into the quenchant, especially when manual quenching must be used. The part must enter the quenchant smoothly. (In the jargon of heat treaters, avoid having the part “slap” the quenchant.) Uniform heat transfer throughout the part will prevent cooling differences and strain differences. Changes in heat transfer in the horizontal direction are usually more detrimental than changes in the vertical direction. It is important to maintain the quenchant at the proper temperature, control its ramp-up, ensure its uniform flow, select the most appropriate quenchant (e.g., air, water or polymer), etc. For example, the cooling rate of the polymer can be adjusted for the needs of a specific application by varying the concentration, temperature and stirring intensity to ensure uniform heat transfer and quench rate during the boiling phase of the bubble nucleus. Maintenance of the quenchant is also important. For parts with complex shapes, such as forgings, castings, impact extrusions and parts made from thin plates, lower quench rates can be used to improve deformation behavior.
- Surface peeling/high-temperature oxidation: This issue is discussed in detail in the February 2016 edition of Industrial Heating in the Heat Treatment Problem Diagnosis section, “High-temperature Oxidation – Case Studies.
- Over-aging: This can cause a loss of mechanical properties. If the aging temperature is too high and/or the aging time is too long, the critical nucleus size of the precipitated phase in the supersaturated solid solution will increase, resulting in a lower strength index after aging.
- Under-aging: This may also result in loss of mechanical properties.
- Improper natural aging: The length of natural aging varies from about 5 days for 2xxx series alloys to about 30 days for other alloys. 6xxx and 7xxx series are more unstable at room temperature, and changes in mechanical properties can last for many years. There are some alloys where natural aging is suppressed or delayed for several days after a low temperature treatment of -18°C (-1˚F) or less. The usual practice is to have finished forming, straightening and stamping before changing the material properties by aging. For example, cryogenic treatment is a common practice for 2014-T4 rivets to maintain good riveting performance.
- Improper Artificial Aging: Artificial aging (also known as precipitation heat treatment) is a longer, lower temperature process. Temperature control is critical, and a temperature uniformity of ±6˚C (±10˚F) must be strictly guaranteed. The optimum target for temperature uniformity should be ±4˚C (±7˚F).
- Insufficient holding time: The consequence is that the desired mechanical properties are not achieved. Too short a time will result in insufficient supersaturation, while too long a time will tend to distort the part.
- Poor temperature uniformity: This can result in failure to achieve or even change the mechanical properties. Typical requirements for process temperature uniformity are ±6˚C (±10˚F), while most aerospace applications expect ±3˚C (±5˚F).
- Improper cold working after solid solution treatment: This is usually due to a lack of understanding of the reaction of the alloy being treated. For example, cold working of a quenched 2xxx series alloy will significantly increase its response to subsequent precipitation treatments.
- Inadequate cooling rate during solution heat treated product annealing: The maximum cooling rate must be maintained at 20˚C (40˚F) per hour until the temperature is reduced to 290˚C (555˚F). The cooling rate below this temperature is less important.
Source: Network Arrangement – China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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