Hardening

The term hardening is often used in the commercial heat-treating industry to describe the process otherwise known as quench and tempering. Steel is heated to an austenitizing temperature, then quenched in oil or water whereupon it is subsequently tempered to increase toughness. The goal of hardening/quench and tempering is to produce a high hardness and strength in a steel part, so that the material has the required engineering properties called for by the design. Cincinnati Steel Treating performs hardening operations on many different types of steel, and we use many different types of processes for hardening.

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Hardening Capabilities at Cincinnati Steel Treating (CST)

Part being Quenched in OilCincinnati Steel Treating (CST) has many different options for hardening/quench and tempering. There are car bottom furnaces for quench and tempering large parts, internal quench furnaces for quench and tempering in a carbon controlled atmosphere, pit furnaces for hardening long/narrow parts,

and small box furnaces for heat treating small parts. CST has the capability of quench and tempering parts in air, oil, water, or a polymer mixture. Some of the sizes and weights of parts that can be hardened/quench and tempered are shown in Table #1. Consult with us on your application and allow our expertise to benefit you in your heat treating needs.

Table #1 – Furnaces used for Hardening/Quench and Tempering at CST

Furnaces

Dimensions

Maximum Weight per load (lbs)

Large Internal Quench

48” X 36” X 72”

3500

Small Internal Quench

30” X 30” X 48”

1200

Pit Furnace # 2

24” diameter X 50” deep

1000

Pit Furnace # 8

18” diameter X 120” deep

1000

Hardening/Quench and Tempering Applications

CNC milled parts isolated on whiteSteel is considered hardened when it undergoes the austenite-to-martensite solid state phase transformation. This is accomplished by heating the steel part above the austenite transformation temperature and then quenching it in a fast cooling medium such as water, oil, or air. The strengthening

occurs when the microstructure changes from austenite to martensite. Martensite is a meta-stable phase that has a greater strength and hardness compared to either room temperature ferrite or high temperature austenite. The degree of strengthening in the martensite is dependent upon the carbon content of the steel. The hardening/quench and tempering operation is finished when the part is heated to an intermediate temperature in order to impart toughness on the part, i.e. it is tempered.

Many different types of parts are hardened/quench and tempered. The list includes gears, structural component parts, tie-rods, shafts, spindles, housings, pins, couplings, brackets, and energy absorbing members of car chassis. Often a hardening operation is performed before a second heat treating operation such as induction hardening or nitriding. The hardening produces a high strength “core” in the part that can lead to the production of a deeper case in the nitride/induction hardened part.

The essential element that must be present in steel when choosing to perform a hardening/quench and tempering operation is carbon. Carbon gives martensite strength through the carbon atoms that occupy interstitial sites in the iron lattice of the part. The higher the carbon content, the greater the hardness that can be achieved. Commonly, medium carbon alloy steels are chosen for hardening/quench and tempering operations because the presence of the alloy allows for a greater depth of hardness. This depth of hardness allows larger cross sections to be heat treated to higher hardnesses. A medium carbon alloy steel part can therefore be given a high strength and toughness that will exist throughout the cross section, thus allowing it to meet greater engineering requirements. Consult with one of our engineers to get a recommendation for the best grade of steel to use for your application.

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