Quenching Review - Heat Treating

Metals and Materials Table of Contents

Quenching - heat treating is the process of rapidly cooling metal parts from the solution or austenitizing treating temperature, typically from within the range of 815 to 870 °C (1500 to 1600 °F) for steel. High-alloy and stainless steels may be quenched to minimize grain boundary carbides or to improve the ferrite distribution but most steels including carbon, low-alloy, and tool steels, are quenched to produce controlled amounts of martensite in the microstructure. Successful hardening usually means achieving the required microstructure, hardness, strength, or toughness while minimizing residual stress, distortion, and the possibility of cracking.

The a quench medium is usually a liquid and depends on the harden-abilityof the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired microstructure. The most common quenchmedia are either liquids or gases. The liquid quenchcommonly used include:

• Oil that may contain a variety of additives
• Leceted liquid polymers
• Water that may contain salt or caustic additives

Gas type quench medians may also be used, such as inert gases including helium, argon, and nitrogen. These quench gasses are sometimes used after austenitizing in a vacuum.

The ability of a quenchto harden steel depends on the cooling characteristics of the quenching medium. Quenching effectiveness is dependent on the steel composition, type of quench or the quenchuse conditions. The design of the quenching system and the thoroughness with which the system is maintained also contribute to the success of the process.

The goal of the quenching process is to cool steel from the austenitizing temperature quickly enough to form the desired micro-structuralphases, sometimes bainite but more often martensite. The basic quenchfunction is to control the rate of heat transfer from the surface of the part being quenched.

Quenching Process
The rate of heat extraction by a quenching medium and the way it is used substantially affects quenchperformance.
Variations in quenching practices have resulted in the assignment of specific names to some quenching techniques:

• Direct quenching
• Time quenching
• Selective quenching
• Spray quenching
• Fog quenching
• Interrupted quenching

Quench Equipment: There are three types of furnaces that are commonly used in quench hardening: salt bath furnace, continuous furnace, and box furnace. Each is used depending on what other processes or types of quench hardening are being done on the different materials.