Cryogenic Hardening 21465l

CRYOGENIC HARDENING

CONTENTS • Introduction • Process of cryo treatment • Austenitising • Quenching • Cryo treatment • Tempering • Metallurgical aspects • Benefits of the cryogenics • Applications

INTRODUCTION The word, “Cryogenics” is taken from two Greek words “kryos” which means ‘frost’ or freezing, and “genic” meaning to ‘produce’ or generated. Cryogenic hardening is a Cryogenic heat treating process. Heat treatment is a combination of heating and cooling applied to a metal or alloy. Where the material is cooled to approximately -185 c. Whenever material is subjected to any manufacturing operation, it is subjected to stresses.

Process of Cryo-Heat-Treatment .

Austenitising Austenitization means to heat the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite. During austenitising, the final alloying element partitioning occurs between the austenite matrix (that transforms to martensite on cooling) and the retained carbides.

Quenching After Austenitising temperature,the steel is cooled to ambient temperature rapidly in a suitable quenching media like water,oil,air. Once the austenite is cooled below its critical temperature,it becomes unstable and it starts to transform in to martensite After CHT there would always be some retained austenite in the steel.Which is up to 20-30%.

Cryotreatment Cryogenic Treatment (CT) of tool materials consists of three stages 1) cooling of tool material fromroom temperature, at an extremely slow rate ranging from0.5 to 1 C/min, 2)Shallow Cryogenic Treatment (SCT) for the period of -84 C 3) Deep Cryo-genic Treatment (DCT) for the period of -196 C

Tempering Tempering is the process of reheating the steel at predetermined temperatures which is lower than the transformational temperature  Martensite precipitates fine carbides, which are named as transition carbides. Nucleation of these carbides relieves micro-stresses inthe brittle primary martensite and prevents micro cracking on surface of the steel

Metallurgical Aspects Transformation Martensite

of

Retained

Austenite

to

- After CHT there would always be some retained austenite in the steel.Which is up to 20-30%. - The retained austenite as a soft phase in steels could reduce the product life, it can be transformed into martensite

Precipitation of Fine η- Carbides Deep cryogenics is due to an enhancement of the precipitation of fine eta-carbides during the subsequent temper. -The strain energy in the martensite lattice increases at a lower temperature. As a consequence Carbon atoms migrate and form a clusters. - During the subsequent heating back to the room temperature or even a tempering, these clusters act as nuclei for the formation of the ultra fine etacarbides.

Cryogenic agent A "cryogenic agent" is an agent (solid, liquid or gas) that can be used to cool an object to cryogenic temperatures, namely temperatures below below –150 °C (–238 °F).

Benefits of Cryogenics • Promotes a more uniform micro-structure • Reduces abrasive and adhesive wear • Permanently changes the structure of the metal resulting in improved machining properties • Improved surface finishing • Reduce the cost of tool remanufacture

Applications • Gun barrels: increases the wear life of the barrel and makes cleaning easier and faster. • Grinding: allows a better cut, less, a better finish, and less tensile residual induced into the work piece. • Machining: lathes, drill bits, cutting and milling tools • Dental and Surgical Instruments • Engine parts:

Conclusion Cryogenic hardening is a permanent, nondestructive, non-damaging process, which reduces abrasive wear , relieves internal stress, minimizes the micro cracking due to shock forces, lengthens part life, and increases performance.

References www.sciencedirect.com/ en.wikipedia.org/wiki/ScienceDirect Material science and metallurgy- Kestoor praveen

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