Annealing, normalizing, quenching, quenching and tempering... Are you clear about these heat treatments? The role of heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metals. According to different purposes of heat treatment, heat treatment process can be divided into two categories: preliminary heat treatment and final heat treatment.

1、 Preliminary heat treatment

The purpose of preliminary heat treatment is to improve processing performance, eliminate internal stress and prepare good metallographic structure for final heat treatment. Its heat treatment processes include annealing, normalizing, aging, quenching and tempering, etc.

1) Annealing and normalizing

Annealing and normalizing are used for hot worked blanks. Carbon steel and alloy steel with carbon content greater than 0.5% are often annealed to reduce their hardness and easy to cut; For carbon steel and alloy steel with carbon content less than 0.5%, normalizing treatment is adopted to avoid sticking to the tool when cutting with too low hardness. Annealing and normalizing can refine the grain size and uniform the structure, and prepare for the future heat treatment. Annealing and normalizing are usually arranged after blank manufacturing and before rough machining.

2) Aging treatment

Aging treatment is mainly used to eliminate the internal stress generated in blank manufacturing and machining. In order to avoid excessive transportation workload, for parts with general accuracy, an aging treatment can be arranged before finishing. However, for parts with high accuracy requirements (such as the box of coordinate boring machine), two or more aging treatment procedures shall be arranged. Simple parts may not be subject to aging treatment.

In addition to castings, for some precision parts with poor rigidity (such as precision lead screws), in order to eliminate the internal stress generated during machining and stabilize the machining accuracy of parts, multiple aging treatments are often arranged between rough machining and semi finishing machining. For the processing of some shaft parts, aging treatment shall also be arranged after the straightening process.

3) Tempering

Quenching and tempering is to conduct high-temperature tempering treatment after quenching. It can obtain uniform and fine tempered sorbite structure and prepare for reducing deformation during surface quenching and nitriding treatment in the future. Therefore, quenching and tempering can also be used as preparatory heat treatment.

As the comprehensive mechanical properties of the parts after quenching and tempering are better, some parts with low requirements for hardness and wear resistance can also be used as the final heat treatment process.

2、 Final heat treatment

The purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance and strength.

1) Quench

Quenching includes surface quenching and integral quenching. Among them, surface quenching is widely used because of its small deformation, oxidation and decarburization. Moreover, surface quenching also has the advantages of high external strength and good wear resistance, while maintaining good internal toughness and strong impact resistance. In order to improve the mechanical properties of surface hardened parts, heat treatment such as quenching and tempering or normalizing is often required as the preliminary heat treatment. The general process route is: blanking - forging - normalizing (annealing) - rough machining - quenching and tempering - semi finishing - surface quenching - finishing.

2) Carburizing and quenching

Carburizing quenching is applicable to low carbon steel and low alloy steel. First, the carbon content of the surface layer of the parts is increased. After quenching, the surface layer can obtain high hardness, while the core still maintains certain strength and high toughness and plasticity. Carburizing is divided into integral carburizing and partial carburizing. During partial carburization, anti-seepage measures (copper plating or anti-seepage material plating) shall be taken for the non carburized part. Since the carburizing quenching deformation is large and the carburizing depth is generally between 0.5 and 2mm, the carburizing process is generally arranged between semi finishing and finishing.

The process route is generally: blanking forging normalizing rough and semi finishing carburizing quenching finishing. When the non carburized part of the locally carburized part adopts the process scheme of removing the excess carburized layer after increasing the allowance, the process of removing the excess carburized layer shall be arranged after carburization and before quenching.

3) Nitriding treatment

Nitriding is a treatment method that allows nitrogen atoms to penetrate into the metal surface to obtain a layer of nitrogen-containing compounds. Nitriding layer can improve the hardness, wear resistance, fatigue strength and corrosion resistance of parts. Since the nitriding treatment temperature is low, the deformation is small, and the nitriding layer is thin (generally not more than 0.6 ~ 0.7mm), the nitriding process should be arranged as late as possible. In order to reduce the deformation during nitriding, it is generally necessary to conduct high-temperature tempering to eliminate stress after cutting.

annealing

3、 Issues related to heat treatment

For the heat treatment of aluminum, many problems reported include:

Improper placement of parts - this can lead to deformation of parts, in large part because the quenching agent cannot transfer heat quickly enough to obtain the desired mechanical properties. Improper placement may also cause thermal deformation (because the creep strength of aluminum is not large enough). Proper placement can avoid these problems.

Heating / heating too fast - this will cause thermal deformation and should be prevented. Proper placement of components helps to heat evenly.

The residual stress level is higher than expected - heat treatment not only changes the mechanical properties, but also directly affects the residual stress level. The following are some possible reasons: there is a large difference in the cooling speed between the surface and the interior during quenching (including cooling after solidification of the casting); Improper temperature rise speed; The temperature changes in the intermediate step; wait. The residual stress is related to the (large) difference of cooling speed, the sudden change of section thickness and section size of components and the strength of materials. Remember that the stress caused by quenching is much greater than that caused by other processes, including casting.

Time / temperature / quenching parameters fluctuate - they will lead to differences in mechanical and / or physical properties between different parts and between different batches. The reasons include too long transfer time of parts, improper quenching (too slow), excessive heating, insufficient heating or changes in time temperature parameters during precipitation hardening. For example, when the time is too long and the temperature is too high, large particles (precipitates) will be precipitated.

Excessive heating - it is easy to produce primary melting or eutectic melting (Fig. 2). For example, the temperature of solution heat treatment is close to the melting point of many aluminum alloys (especially 2XXX series, which is often only a few degrees lower than their melting point). In order to promote the dissolution of solid alloy elements, an appropriate temperature is required.

Insufficient heating - this will 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, it is not easy to form solute atom aggregation region (GP region), resulting in low strength after aging.

Deformation caused by insufficient quenching - the problem / difficulty in this aspect is the action of parts entering the quenching agent, especially when manual quenching is required. The parts shall enter the quenching agent smoothly. (in the jargon of the heat treatment personnel, it is necessary to avoid "beating" the quenchant by the parts.) Uniform heat transfer of the whole part can prevent cooling difference and strain difference. Changes in heat transfer in the horizontal direction are generally more unfavorable than changes in the vertical direction. It is very important to keep the quenchant at a proper temperature, control its temperature rise, ensure its uniform flow, and select a more suitable quenchant (such as air, water or polymer), etc. For example, according to the needs of a specific application, the cooling speed of the polymer can be adjusted by changing the concentration, temperature and stirring intensity, so as to ensure uniform heat transfer and quenching speed in the nucleate boiling stage. Maintenance of Quenchant is also important. For parts with complex shapes, such as forgings, castings, impact extrusion parts and parts made of thin plates, lower quenching speed can be used to improve the deformation behavior.

Surface peeling / high temperature oxidation - this problem is discussed in detail in the heat treatment problem diagnosis column "high temperature oxidation - Case Study" of the February 2016 edition of industrial heating.

Over aging - this may cause loss of mechanical properties. If the aging temperature is too high and / or the aging time is too long, the critical crystal nucleus size of the precipitated phase in the supersaturated solid solution will increase, and the strength index after aging will decrease.

Insufficient aging - this may also result in loss of mechanical properties.

Improper natural aging - the time of natural aging varies, for example, about 5 days for 2XXX series alloys and about 30 days for other alloys. 6xxx and 7xxx series are relatively unstable at room temperature, and the change of mechanical properties will last for many years. There are some alloys after - 18 ° C (- 1 ? F) Or below, the natural aging will be suppressed or delayed for several days. It is common practice to complete forming, straightening and stamping before changing the material properties through aging. For example, low temperature treatment is a measure often taken by 2014-t4 rivets to maintain good riveting performance.

Improper artificial aging - Artificial Aging (also known as precipitation heat treatment) is a process with long time and low temperature. Temperature control is very important and must strictly ensure ± 6 ? C(±10 ? F) Temperature uniformity. The preferred target for temperature uniformity should be ± 4 ? C(±7 ? F)。

Insufficient holding time - the result is that the desired mechanical properties are not achieved. Too short a time will lead to insufficient supersaturation, while too long a time will easily lead to deformation of components.

Poor temperature uniformity - this can lead to failure or even change in mechanical properties. Typical requirements for process temperature uniformity are ± 6 ? C(±10 ? F) While most aviation applications want to achieve ± 3 ? C(±5 ? F)。

Improper cold working after solution treatment - this is usually due to a lack of understanding of the reaction of the alloy being treated. For example, the cold processing of the quenched 2XXX series alloy will significantly increase its reaction to subsequent precipitation treatment.

The cooling rate of solution heat treatment products is not enough during annealing - the cooling rate must be kept at 20% per hour ? C(40 ? F) Until the temperature drops to 290 ? C(555 ? F)。 The cooling rate below this temperature is not very important.

Source: Shenyang Zhonglian copper and aluminum industry Co., Ltd