Annealing-Quenching-Tempering

One. Types of Annealing

1. Complete annealing and isothermal annealing

Complete annealing, also known as recrystallization annealing, is generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled profiles of various carbon steels and alloy steels with sub-co-analysis components, and sometimes for welded structures. Generally, it is often used as the final heat treatment of some non-heavy workpieces, or as the pre-heat treatment of some workpieces.

2. spheroidizing annealing

The spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as the steel used in the manufacture of cutting tools, measuring tools and molds). Its main purpose is to reduce the hardness, improve machinability, and prepare for later quenching.

3. Stress relief annealing

Stress relief annealing is also called low temperature annealing (or high temperature tempering), which is mainly used to eliminate the residual stress of castings, forgings, weldments, hot-rolled parts, cold-drawn parts, etc. If these stresses are not eliminated, it will cause deformation or cracks in the steel parts after a certain period of time, or in the subsequent cutting process.

II. When quenching, the commonly used cooling media are brine, water and oil. The workpiece quenched by brine is easy to get high hardness and smooth surface, and it is not easy to produce soft points that are not hardened, but it is easy to deform the workpiece seriously and even crack. The use of oil as a quenching medium is only suitable for the quenching of some alloy steel or small-sized carbon steel workpieces with relatively large stability of undercooled austenite.

Three. Purpose of steel tempering

1. Reduce brittleness, eliminate or reduce internal stress, there is a lot of internal stress and brittleness after quenching, such as not timely tempering often make steel deformation and even cracking.

2. Obtain the mechanical properties required by the workpiece. The workpiece has high hardness and brittleness after quenching. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted by appropriate tempering, reducing brittleness, and obtaining the required toughness and plasticity.

3. Stable workpiece size

4. For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing), so that the carbides in the steel are properly aggregated and the hardness is reduced to facilitate cutting.

Selection of furnace type

Furnace type should be based on different process requirements and the type of workpiece to decide

1. For those that cannot be produced in batches, the workpiece sizes are not equal, and there are many types, the process is required to be universal,

Multi-use, optional box furnace.

2. When heating long shaft and long screw rod, pipe and other workpieces, deep well type electric furnace can be selected.

3. Small batch of carburizing parts, can choose well type gas carburizing furnace.

4. For the production of large quantities of automobiles, tractor gears and other parts, continuous carburizing production line or box-type multi-purpose furnace can be selected.

5. When heating the blank of stamping parts for mass production, the rolling furnace and the roller hearth furnace are selected.

6. For batch of shaped parts, push rod type or conveyor belt type resistance furnace (push rod furnace or casting belt furnace) can be selected for production.

7. Small mechanical parts such as screws, nuts, etc. can be used for vibrating hearth furnace or mesh belt furnace.

8. Steel ball and roller heat treatment can choose the internal spiral rotary tube furnace.

Non-ferrous metal ingot in mass production can be used when the push rod type furnace, and for non-ferrous metal small parts and materials available air circulation heating furnace.

Heating defects and control

1. overheating

We know that overheating in the heat treatment process can easily lead to coarse austenite grains and reduce the mechanical properties of the parts.

1. General overheating: The heating temperature is too high or the holding time is too long at high temperature, causing austenite grain coarsening is called overheating. Coarse austenite grains will reduce the strength and toughness of the steel, increase the brittle transition temperature, and increase the tendency of deformation cracking during quenching. The cause of overheating is out of control of the furnace temperature meter or mixing (often due to ignorance of the process). The superheated structure can be annealed, normalized or tempered at high temperature for many times, and then re-austenized under normal circumstances to refine the grain.

2. Fracture heredity: a steel with overheating structure, after reheating and quenching, although the austenite grain refinement, but sometimes still appear coarse granular fracture. The theory of fracture heredity is controversial. It is generally believed that the impurities such as MnS are dissolved into austenite and enriched in the crystal interface due to the high heating temperature. When cooling, these inclusions will precipitate along the crystal interface and break along the coarse austenite grain boundary when impacted.

3. The heredity of coarse tissue: when the steel parts with coarse martensite, bainite and Wei's tissue are re-Auschenized, they are heated at a slow rate to a regular quenching temperature, even lower, and their austenite grains are still coarse, which is called tissue heredity. To eliminate the heredity of coarse tissue, intermediate annealing or multiple high temperature tempering can be used.

2. burning phenomenon

If the heating temperature is too high, not only the austenite grains will be coarse, but also the grain boundaries will be oxidized or melted locally, resulting in the weakening of the grain boundaries, which is called over-burning. The performance of steel is seriously deteriorated after over-burning, and cracks are formed during quenching. The burned tissue cannot be restored and can only be scrapped. Therefore, in the work to avoid the occurrence of burning.

3. decarburization and oxidation

When steel is heated, the carbon on the surface layer reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), reducing the surface carbon concentration called decarburization. The surface hardness, fatigue strength and wear resistance of decarburized steel are reduced after quenching, and the residual tensile stress on the surface is easy to form surface network cracks.

When heated, the iron and alloys on the surface of the steel react with the oxygen, carbon dioxide, water vapor, etc. in the medium (or atmosphere) to form an oxide film. The phenomenon is called oxidation. High temperature (generally more than 570 degrees) after the workpiece is oxidized, the dimensional accuracy and surface brightness deterioration, with poor hardenability of the oxide film of steel parts prone to quenching soft spots.

In order to prevent oxidation and reduce decarburization measures are: workpiece surface coating, sealed heating with stainless steel foil packaging, the use of salt bath furnace heating, the use of protective atmosphere heating (such as purified inert gas, control furnace carbon potential), flame combustion furnace (so that the furnace gas is reducing)

4. hydrogen embrittlement

The phenomenon that the plasticity and toughness of high-strength steel are reduced when heated in a hydrogen-rich atmosphere is called hydrogen embrittlement. Hydrogen embrittlement of the workpiece by hydrogen treatment (such as tempering, aging, etc.) can also eliminate hydrogen embrittlement, the use of vacuum, low hydrogen atmosphere or inert atmosphere heating can avoid hydrogen embrittlement.