Heat Treatment

Heat treatment of precision investment castings is a metal processing technology to controlled heating and cooling of metals to alter their mechanical and physical properties without changing the product shape. The technique involves the use of heating or chilling, usually to extreme temperatures, to attain a desired result, such as - hardening or softening of a metal. Some of the common techniques of heat treatment include annealing, case hardening, tempering and quenching. As a matter of fact, the term heat treatment applies only to processes where the heating and cooling is done for the basic objective of altering the properties intentionally. Heating and cooling, however, sometimes occur as incidental phases of other manufacturing methods such as hot forming or welding.

Primarily associated with increasing the strength of material, heat treatment can also be used to change certain manufacturing objectives such as improved machining, improved formability, and to restore ductility after a cold working operation. Hence it is a very enabling manufacturing process, which can not only help other manufacturing methods, but can also enhance product performance by increasing the strength or other desirable characteristics.

The four basic types of heat treatment processes used today are Annealing, Normalizing, Hardening and Tempering.

The techniques used in each method and how they relate to steel workers are given below:

Generally, annealing is the opposite of hardening. Metals are annealed to relieve internal stresses, to soften them, to make them more ductile, and to refine their grain structures. Annealing comprises of heating a metal to a particular temperature, holding it at that temperature for a definite time, and then cooling the metal to room temperature. The cooling process depends on the metal and the characteristics desired. Some metals are furnace-cooled, while others can be cooled by burying them in ashes, lime, or other insulating materials.

Welding produces areas, which have molten metal next to other areas, which are at room temperature. As the weld cools, internal stress occurs along with brittleness and hard spots. Welding can in reality weaken the metal. Annealing is just one of the techniques used for correcting these problems.

A type of heat treatment applicable to ferrous metals only, normalizing differs from annealing in that the metal is heated to a higher temperature, which is then removed from the furnace for air cooling.

The basic objective of normalizing is to remove the internal stresses caused by heat treating, welding, casting, forging, forming, or machining. If not controlled, stress can result in metal failure; hence, before hardening the steel, it should be first normalize to get the desired results. Generally, low-carbon steels do not require normalizing; however, if they are normalized, they do not cause any harmful effects. Usually, the castings are annealed, instead of normalizing; however, some castings require normalizing.

Normalized steels are stronger and harder than annealed steels. In the normalized condition, steel is much sturdier than in any other structural condition. Parts subjected to impact and those, which require maximum toughness with resistance to external stress are generally normalized. In normalizing, the mass of metal has an impact on the rate of cooling and on the resulting structure. Thin pieces cool faster and are sturdier after normalizing than thick ones.

Metal hardening treatment constitutes of heating the steel to a definite temperature and then cooling it quickly by plunging it into oil, brine, or water. Majority of steels require rapid cooling (quenching) for hardening, however a few can be air-cooled with the same results. Although hardening enhances the hardness and strength of the steel, it makes it less ductile. Usually, the harder the steel, the more brittle it becomes. To remove some of the brittleness, the steel should be tempered after hardening.

Many nonferrous metals can be hardened and the strength can be increased by controlled heating and rapid cooling. In this case, the process is called heat treatment, instead of hardening.

After hardening, steel is generally harder than required and is too brittle for most practical applications. In addition, stern internal stresses are set up during the speedy cooling from the hardening temperature. To relieve the internal stresses and to reduce brittleness, the steel should be tempered after it is hardened. Tempering process comprises of heating the steel to a particular temperature (below its hardening temperature), holding it at that temperature for a definite time, and then cooling it, generally instill air. The final hardness, strength, and ductility depend on the temperature to which the steel is heated during tempering.

Primarily, tempering is done to reduce the brittleness imparted by hardening and to generate specific physical properties within the steel. Tempering always follows, never precedes, the hardening process. Besides reducing the brittleness, tempering helps in softening of steel. That is unavoidable, and the amount of hardness, which is lost depends on the temperature, which the steel is heated to during the process of tempering.

Tempering is always exercised at temperatures below the low critical point of the steel. In this regard, tempering differs from annealing, normalizing, and hardening, wherein the temperatures are above the upper critical point.

Precision Investment Casting of Ningbo Investment Casting offers wide selection ways of heat treatment service. With long term professional heat treatment partner, we can provide below heat treatment methods with low cost in below:

1. Normalizing: Heating investment casting blanks to or above critical point temperature and maintain a certain time, cooling in the air to get the pearlitic tissue. Aim of normalizing is to improve the mechanical properties of low carbon steel, improve the cutting process ability, refine the grain size, remove the tissue defects, and ready for further heat treatments. Normalizing is more suitable for engineered carbon steel investment castings and thick, big and complex alloy steel investment castings.

2. Annealing: A kind of heat treatment methods that heating investment castings to proper temperature for a while, and cooling down slowly. Annealing is mainly used to lower the hardness, improve plasticity for further machining, and reduce residual stress of investment castings.

3. Tempering: Improves ductility, crack resistance, malleability, and toughness of products. Through controlled reheating, we are able to quickly reduce the brittleness of investment castings.

4. Quenching: Quenching is a process of cooling investment castings at a rapid rate. This is most often done to produce a martensite transformation. In ferrous alloys, this will often produce a harder metal, while non-ferrous alloys will usually become softer than normal. To harden by quenching, investment castings (usually steel or cast iron) must be heated above the upper critical temperature and then quickly cooled.

Attention: However, most non-ferrous metal castings, like alloys of copper, aluminum, or nickel, and some high alloy steel castings such as austenitic stainless steel investment castings (304, 316), produce an opposite effect when these are quenched: they soften. Austenitic stainless steels investment castings must be quenched to become fully corrosion resistant, as they work-harden significantly.

5. Carburizing Treatment: Carburizing is the process of directing carbon element into the surface of investment castings. It will greatly improve the surface hardness and wear resistance by change the carbon composition of products, but still keep low hardness inside.

6. Tempering and Hardening: Compound heat treatment process by quenching and high tempering of investment castings. This kind of heat treatment way is normally used to improve the both surface and inner hardness of precision investment castings. Especially for those replacement wear parts.

7. Case Hardening: Also referred to as surface hardening, this method infuses hardening elements into the surface of investment castings, thus to provide an extra protection layer.

Above are the main heat treatment ways listed for investment castings. Heat treatment is carried out mainly according to the application of investment castings. If you are not familiar with heat treatment, please feel free to contact us, our engineer will suggest the right way if your products do require this service.

Contact us at sales@nbinvestmentcasting.com once you are in need of our heat treatment service.