HEAT TREATMENT

 

HEAT TREATMENT

 

In order to produce an efficient tool it is necessary not only to select the correct steel but also subject it to a proper and careful heat treatment. Any solid metal normally has a definite infrastructure at a certain energy state. But the chemical as well as physical change will occur by the application of heat. Therefore heat treatment may be defined as the process of heating and cooling of metals or alloys in the solid state to induce desired properties. Some requirements are listed below:

1)           To improve mach inability.

2)           To change or reefing grain size.

3)           To improve magnetic and electrical properties.

4)           To increase resistance to wear, heat and corrosion.

5)           To produce a hard surface on ductile interior.

 

The steps for hardening are:

1)           Stress relieving.

2)           Preheating for hardening.

3)           Heating to hardening temperature and soaking.

4)           Quenching.

5)           Tempering.

6)           Inspection and testing.

 

1)    STRESS RELIEVING

The parts are loaded in a metal box which has hydro carbons like charcoal, cast iron chips etc. which prevent decarborization. Then it is heated steadily from room temperature to 650° C in a muffle furnace. It is soaked for 12 hrs and then furnace is cooled at rate of 50° C/hr. This operation eliminates machining stresses, which may result in cracking during hardening. If heavy machining is to be performed stress relieving is recommended intermittently.

 

2)    PRE-HEATING FOR HARDENING:

It is very important in hardening operation. Parts are heated to 300-500°C and soaked. Then it is transferred to neutral salt bath. Preheating relieves thermal stress induced for having exposed suddenly to hardening temperature.

 

3)    HEATING TO HARDENING TEMPERATURE AND SOAKING

After pre heating, tool parts are transferred into salt bath, which is maintained at a temperature of 820-850°C. Depending upon the mass of the tool parts, each part is soaked. Soaking is essential in order to ensure solid solution state in the tool throughout the phase.

 

4)    QUENCHING

It refers to rapid cooling of material from austenising temperature to room temperature by choosing appropriate cooling media. The usual quenching media for hot die steel is air. Air quenching provides a cooling rate of about 65°C/Sec at the surface. It is the most suitable for quantity tool steels. Still air is normally recommended for better results

 

5)    TEMPERING

The parts should be tempered immediately after quenching (within half an hour). Parts are tempered as per the ensure toughness. Tempering is done in a tempering bath maintained at a temperature of about 150-250°C. After soaking for one and half hours they are allowed cool in air with a little loss in brittleness. After tempering the brittleness of steel reduces with some decrease in hardness, but the toughness and impact strength of steel increases and also the internal stresses due to quenching are relieved in tempering.

 

HEAT TREATMENT PROCESS FOR CASE HARDENING STEEL

Case hardened steels cannot be hardened directly as other steels because of less percentage of carbon transformation of pearlite and ferrite to austenite and marten site is very less. So before hardening a process called carbonizing is done.

 

CARBURISING

It is defined as the method of heat treatment by which carbon content at the surface of ferrous material in increased. To increase the carbon content at the surface of steel following factors to be considered.

1)                Ferrite phase of iron.

2)                Austenite phase of iron.

 

Depending upon the procedure of carburising and state of medium used, they are classified as:

1)                     Pack carburising.

2)                     Gas carburising.

3)                     Liquid carburising.

 

Here the parts to be carburised are placed in containers containing carbonaceous material like charcoal etc. the container is heated to a temperature of about 850°C-925°C. At this temperature diffusion takes place and carbon is absorbed. After soaking time the container is allowed to cool by this the carbon percentage will be increased.

 

HEAT TREATMENT PROCESS FOR OIL HARDENING NON SHRINKAGE STEELS (OHNS)

HARDENING

Since this material contains 1.1% of carbon it can be hardened without any pre-operations like carbourising etc. Here the material is heated to about 850°C, the part is then soaked which helps for structural changes. To retain the martensitic structure quenching in oil suddenly cools the material. The hardness range obtained is about 56-64HRC. The martensite structure will be hard and brittle.

 

ANNEALING:

The steel is annealed for following purposes:

1)           To make steel soft i.e., to reduce hardness and increase mach inability.

2)           To relieve internal stresses.

3)           To obtain microstructure.

4)           To improve magnetic and electric property.

 

The steel is heated uniformly to the austenite phase and then cooled slowly in the furnace where the cooling rate is 50°C/hr and therefore results in the formation of coarse peralite, which is soft.

The annealing temperature for hypo eutectoid is HCT+50°C and for the hypereutectoid steel is LCT+50°C. Because steel when heated above the critical range consists entirely of austenite, transforms into peralite with the free constituents of cementite, which is hard. The presence of cemetite makes the steel hard.

 

At the temperature LCT+50° C, the formation of austenite will still be in progress and after cooling results in the formation of lesser amount of cementite. Therefore steel becomes much softer.

 

TEMPERING

The material is heated to a temperature of about 150°C -300°C. The tempering time depends on the temperature. There the temperature is inverse of the time, but tempering time depends on the size of the material to be tempered. The hardness is reduced by 3-6 HRC.

 

After tempering time, material is brought to room temperature by air quenching.

 

HEAT TREATMENT PROCESS FOR ORVAR SUPREME

This is most important because of its application for core and cavity. Here the material is heated to a temperature range of 980°C to 1040°C. Soaking time is allowed till the required structural changes are done. After soaking the parts are cooled in furnace up to 600°C. That is below critical temperature point. Then it is air quenched to obtain 46-48HRC hardness.

 

TEMPERING.

The material is heated to about 550°C -590°C. Tempering time depends upon the size of the material. Then it is allowed to cool in open air. The hardness may be reduced by 3-6 HRC and eliminating the internal stresses developed during hardening induces toughnes.

 

MAR TEMPERING.

After hardening surface becomes hard and brittle. The center of the piece transfers and expands often cracking the surface of martensite. This happens due to rapid quenching so they are called as quench cracks. This process produces fully martensitic structure with least internal residual stresses distortion and quenches cracks. When the steel reaches the austenitic range is followed by lead alloy bath maintained with same temperature at both core and the surface about 180°C 300°C but not long enough to bainite formation.

 

MULTI TEMPERING.

Also multi tempering at temperature 500°C-650°C should always be employed to ensure the total transformation of austenite to martensite. This has proved advantageous for large or sharp cornered tools.

 

STABILIZING.

Generally these steels tend to retain sustained amount of austenite in the hardened state. This is due to high quality content and is apparent by the low martensite formation temperature, generally below zero sub zero treatment in refrigerator at –75°C or in liquid nitrogen at -196°C is recommended.

CASE HARDENING

After hardening only core remains soft that is, only core will be in cementite (soft) structure and the surface will have marten site (hard) structure. Thus the items will have soft core and hard surface. The parts will be tough hard and wear resistant.

 

TYPES OF CASE HARDENING:

Nitriding.

Cyaniding.

Carburising.

 

NITRIDING: -

This is a process of case hardening resulting from the presence of iron nitrites and alloy nitrites within the steel. Special steels containing small amount of aluminum and chromium, molybdenum are used for nitriding.

 

Here the steel is heated in a closed furnace containing ammonia gas, which partially dissociates at the surface of the hard steel, forming N2 and H2. The N2 diffuses into the surface forming nitrites, which are hard and wear resistant. Quenching is not necessary is an added advantages.

 

This is a costly process and time consuming. But the nitrated surfaces are very hard (62-67HRC), very good wear resistant, high fatigure strength improved corrosion resistance.

 

2)      CYANIDING

It is also called as liquid carbonitriding. It is a process by which the carbon and nitrogen content at the surface of steel is increased.

 

Commonly used cyanide bath consist of sodium chloride, sodium cyanide and sodium carbonate. These Na2Co3 and Nacl are used to provide fluidity and to control the melting point of bath (540 to 620°C).

 

Here cyanide decomposes in the presence of O2 and at the surface of the bath to produce Sodium Cyanide, which in turn decomposes to form nitrogen and carbon as per the reactions shown below.

2NaCN + O2 ----------  2NaCNO

4NaCNO + 2N ---------  Na2Co3 + 2NaCN

2Co------------ Co2 +C

2NaCN + 2CO2---------- 2NaCNO + O2

 

Both are operated in the range of 760- 870°C with an immersion time of 30 to 100 minutes and temperature varies directly to the case hardness. Here the case depth varies from 0.025 to 0.25 depending upon temperature and time of immersion. It is used to produce file hard, wear resistant surface of the steel. It is less costly then liquid carburising but the ingredients are highly poisonous. So care should be taken while handling.

 

Carburising is explained in heat treatment process of case hardening steel.