heat treatment of gauge materials

8/3/2019 Heat Treatment of Gauge Materials

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WHAT IS LIMIT GAUGE ?

Limit gauges are designed to check a singledistance, length or diameter, or may serve theinspection of multiple dimensions. This later

may comprise contours, a combination of contours and length dimensions, geometricforms individually or combined with length,such as tapers and cylinders, or the

interrelation of several forms, sizes and theirlocation on a single body or within aassembly.



WHAT IS HEAT TREATMENT ?

Heat treatment is a group of industrial and metalworking process used to alter the physical, andsometimes chemical, properties of a material. Themost common application is metallurgical. Heattreatments are also used in the manufacture of many other materials, such as glass. Heattreatment involves the use of heating or chilling,normally to extreme temperatures, to achieve a

desired result such as hardening or softening of amaterial. Heat treatment techniques includeannealing, case hardening, precipitationstrengthening, tempering and quenching.



NEED OF HEAT TREATMENT:

Why it is required to perform heat treatment

on gauge materials ?

To maintain wear resistance property.

Dimensional stability of gauge material.

To attain maximum hardness.

To prevent presence of more percentage of austenite in finished gauge.

To increase the service life of gauges.



HEAT TREATMENT OF GAUGE MATERIAL:

There are mainly three types of heat treatment

processes performed on gauges:

1. Sub-zero treatment2. Nitriding

3. Chromium plating

All the above mentioned heattreatment processes are further described in

details.



SUB-ZERO TREATMENT:

The presence of austenite in hardened steel leads to the following defects

a) Maximum hardness can not be obtained.

b) Change in dimension occurs.

c) It is non-magnetic.

Therefore the gauges are cooled below zero degree centigrade temperatureto remove the austenite. Therefore this treatment is called sub zero treatment.Generally the steel is cooled to -78 degree Celsius with the help of solid carbondioxide or dry ice. The coolants like ethylene, ethane and Freon are used to coolthe gauges to still more lower temperatures. The gauges are kept at thistemperature for short period and immediately taken out. Generally the coolingand holding at that temperature take half to one hour time. The sub-zerotreatment should be immediately after hardening the gauge. Therefore the whole

process is carried out in following sequence:a. Hardening

b. Lower temperature cooling

c. Tempering



Fig:- Freezer for sub zero

treatment



NITRIDING:

The steel absorbs nitrogen during this process. Thisprocess takes more time as 0.1 mm thick nitriding layer takesthe time of 10 hours approx. at 500 to 540 degree Celsius.This process is carried out in two stages:

(a) In first stage gauge is held at a temperature of 500to 540 degree Celsius for about 8 to 12 hours. During thisstage 18 to 25% ammonia gets decomposed.

(b) In second stage it is held at 560 degree Celsiustemperature for a period proportional to the requiredthickness of coating. After nitriding is over , the gauge isslowly cooled down to 200 degree Celsius. During thiscooling 20% of nitrogen is decomposed. Surface hardness of 60 to 70 rockwell can be obtained by nitriding. The nitridingof low carbon steel gauge is carried out to increase itsresistance to corrosion.



Fig:- Nitriding process



Fig:- Nitriding furnace



CHROMIUM PLATING:

Chromium plating provides a layer of chromium on gaugesurface. It provides the properties like corrosion resistance,acid resistance, hardness and wear resistance. The smoothand attractive surface of the gauge is provided by thistreatment. There are many methods of chromium plating. Ingas coating method the mixture of hydrogen and chloridevapour is passed, the furnace heated to a temperature of 1000degree Celsius, in which the chromium or ferrous chromiumis kept. Due to chemical reaction of vapour mixture with thissubstances chromium dichloride gas is produced, which

reacts with the gauge metal and helps in obtaining requiredlayer of chromium on the gauge surface. This is a fastprocess. The chromium coated low carbon steel gauges arethen hardened and tempered for improving their mechanicalproperties.



Fig:- Chromium plating



Fig:- Some chrome plated

limit gauges

heat treatment of gauge materials

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