HEAT TREATMENT PROCSSES: Hardening, Tempering, Austempering and Martempering


1. Hardening: 

Process: The operation of hardening is carried out in three stages: 

(i) Heating the work to a temperature critical temp. Hypoeutectoid steels (less than 0.8% carbon) are heated about 30°-50°C above upper critical temperature and Hyper-eutectoid steels (more than 0.83% carbon) are heated above lower critical temp. 

(ii) Holding or soaking the steel at this temperature for a considerable time so that structure may change to austenite. 

(iii) Quenching or cooling at a faster rate in a suitable medium. Water, oil or salt bath may be used as quenching media. Cooling at a rate higher than critical cooling rate enable the austenite to change into martensite. 

Purpose: Purpose of hardening is to 

(i) develop high hardeness to enable it to cut other metals, 

(ii) enable it to cut other metals, 

(iii) to resist wear and abrasion.

Applications: This process is applied all tools and machine parts used for heavy duty work. Hardening may be applied on parts made of medium carbon steels, high carbon steels and alloys steels. 

In alloy steels, the martensite structure may be obtained by slow cooling rate (i.e. cooling in air blast).

 The carbon steels having less than 0.3% carbon does not respond to hardening process. 

Limitations: Hardened steel is in a stressed condition: It may develop cracks. It is also very brittle. Therefore can’t be used directly for practical purposes. After hardening, steels must be tempered to, remove these defects.



Purpose : After completion of hardening heat treatment process, steel develops non-visible micro cracks. It is severely strained due to’ internal stresses (residual stresses). That is why tempering is an essential operation performed after hardening. 

Thus tempering serves the following objects or purposes: 

(i) It reduces hardness and brittleness ‘ . ‘ 

(ii) Intereases ductility, toughness and shock resistance 

(iii) Stabilize the structure of metal 

(iv) Relieve internal stresses produced during hardening 

(v) Eliminate microcracks developed during quenching/ 


Besides above desired properties tempering may result in (i) reduction of hardness (ii) reduction of impact strength and tensile strength 

Process: Tempering process requires or consists of 

(i) reheating the hardened steel to a certain temperature below lower critical temperature followed by:

 (ii) holding or soaking it at this temperature for a considerable time 

(iii) Cooling at a slow rate 


Types of tempering processes: 

Based upon reheating temp range the tempering process may be divided into three types as shown, in the diagram below: 

(i) Low Temperature Tempering: It is done in the temp range of 150oC to 250°C. The purpose of this process is to reduce internal stress and to increase ductility without appreciable loss in hardness. Applied: – for tempering of carbon steel and low alloy cutting tools and measuring tools. 

(ii) Medium Temperature Tempering: It is done in the temp, range of 350°C to 450°C. At this temp martensite is completely converted to troostite. Ductility and toughness of steel is increased. But some hardners and strength are reduced. 

Applied :- for tempering articles subjected to impact load: Chisels, hamers, laminated springs and helical springs etc. : 

(iii) High Temp, tempering: In this process-the hardened steel is reheated between 500 C and 650 C Martensite is completely transformed into sorbite, It eliminates the internal stresses completely. 

Applied: – for tempering machine parts subjected to high stresses e.g. shafts, gears, cranks shaft etc.



 Austempering process consist of 

(i) heating the steels to the hardening temp. 

(ii) holding it there for some time. 

(iii) quenching in salt bath down to a temperature between 300-500°C, so that austenitc is completely converted into lower Bainite (or acicular Troostite). 

(iv) It is then allowed cool in any medium to room temp. 

Characteristics: This process is based on the transformation of austenite to acicular troostite at constant temp. 

• The steel after austempering has almost same hardness of as that of martensite 

• But it is tough and more ductile than other tempered steels 

• Cracks are eliminated 

• Distortion are also eliminated The process is applied for aircraft engine parts



This is a hardening method, known as Martemporing or stepped quenching. 

The process consists of— 

(i) heating the steel to the hardening temp range (austenite range), 

(ii) held at this temp for some time 

(iii) quenching in salt bath having the temp below 300°C 

(iv) holding it at the above temp so that steel attains uniform temp throughout the mass, 

(v) cooling in air to room temp. During this cooling period austenite is converted into martensite and some oustenite 


(i) Less distortions and wrapping because the transformation of austenite to martensite occur simultaneously throughout the mass of object 

(ii) Less volume changes 

(iii) Lesser chances of cracks & internal stresses Applications: It is applied in hardening thin articles of carbon steels and considerably thicker parts of alloy steels

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