MARTEMPERING/MARQUENCHING: MARTEMPERING/MARQUENCHING In this the steel is cooled rapidly by a rate more than the CCR to temp between the nose and Ms, soaked at this temp for sufficient time for equilisation of temp but not long enough tom permit the formation of bainite.then cooled up to room temp in air or in oil Applicable to high c steel and low alloy steel The martensite formed by this process has the following advantages Results in less distortaion and warping since martensite formation starts at the same time throught the material Less possibility of quenching cracks in the component. MARAGING: MARAGING These steels can be air hardened by martensitic transformation and subsequently precipitation hardened by aging hence called maraging These steels low C steels PROCESS: Heated upto austenitic temp.to make austenitic structure from surface to core then held there for 1 hr per 25mm thickness or dia Then it is cooled in air up to room temp. The martensite is soft and tough rather than hard and brittle It can be cold worked to a high degree Steels are then aged at about 500c During aging strain induced precipitation hardening occurs due to precipitation of Ni3TiAl and Ni3Mo phases The structure is very fine precipitates in the martensite matrix Fig. MECHANISM OF HEAT REMOVAL DURING QUENCHING: MECHANISM OF HEAT REMOVAL DURING QUENCHING The mechanism of heat removal during cooling is in 3 stages Vapour blanket cooling stage: the outer surface cools faster than the core The temp of the metal is so high that quenching medium gets vapourised at the surface of the metal and a thin stable film of vapour surrounds the hot metal.
Martempering and Austempering of steel Steel and Cast Iron Martempering and Austempering of steel. Martempering and Austempering of steel Steel and Cast Iron Martempering and Austempering of steel. Luigi Mazzucco.
Cooling rate is relatively slow in this stage. Vapour transport cooling stage:starts when metal is cooled to a temp where the vapour film is no longer stable When temp falls away the vapour film breakes and liquid comes in contact with the hot metal Due to this violent boiling occurs and metal cools rapidly Fastest stage of cooling and martensite formation occurs in this stage 3.
Liquid cooling stage:This starts when the temp of the metal reaches the boiling point of liquid Cooling is by conduction and convection through the liquid The cooling rate is slowest in this stage It is desirable to have martensite formation during this stage so that distortion and cracking will be least Fig. QUENCHING MEDIUMS: QUENCHING MEDIUMS Ideal cooling medium should have fast initial cooling to avoid nose of TTT dia Followed by slow cooling to avoid distorsion and quench cracks No liquid satisfies these conditions like water is having fast initial as well as final cooling rate while conventional oils show slow rate of cooling In order of decreasing cooling rates the liquids are Fig. 4.24 Water solution of 10% NaCl Tap water Fused liquid salt Soluble oil and water solutions Oil Air With Brine quenching hardness up to 60 HRC Oil quenching gives about 28HRC.
VARIABLES AFFECTING HARDENING OF STEEL: VARIABLES AFFECTING HARDENING OF STEEL Hardness of steel is affected by Austenetising temperature: Steel heated to optimum austenetising temperature If lower temp. Lower hardness If higher temp lower hardness due to coarse grain structure having more retained austenite 2. Holding time: For complete transformation of austenite proper holding time holding time increases with thickness or diameter 3. Delay in quenching: results in reduction in hardness of steel due to partial transformation of austenite to other phases like pearlite,ferrite etc 4. Type of quenching medium: transformation to martensite from austenite should be as fast as possible otherwise other phases will get formed like pearlite, bainite etc 5. Temperature of quenching medium: if temp of quenching medium is increased hardness decreases. HARDENABILITY OF STEEL: HARDENABILITY OF STEEL Jominy end quench test Grossman’s test Factors affecting hardenability: Mean composition of steel: All elements increase hardenability except cobalt The homogenity of austenite:The regions of low C content will have low hardenability so more uniform composition gives more hardness.
The grain size of Austenite: Finer the grain more chances of pearlite but if coarse grained austenite is used it gives brittle martensite so undesirable. Undissolved Carbides and nitrides in the austenite:undissolved carbides reduces hardenability. RETAINED AUSTENITE: RETAINED AUSTENITE The austenite which is remained after the transformation to martensite is completed is called as retained austenite. The amount of retained austenite varies from surface to core as surface cools faster than the core Greater the temp difference between the surface and the core higher is the chances of retained austenite at the core. EFFECT OF RETAINED AUSTENITE: 10% retained austenite acts as a cushion for thermal shock during quenching Steels having about 30-40% retained austenite can be cold worked without cracking Hardness of the hardened steel gets reduced due to retained austenite During plastic deformation retained austenite gets transformed to martensite which introduces stresses in steel. If the retained austenite gets transformed to bainite then there will be linear expansion which is not desirable for precision instruments Cant be used for tool steels. TEMPERING: TEMPERING During tempering, martensite tries to transform to ferrite cementite mixture so strength and hardness falls progressively and ductility and toughness increases Higher the temp more chance of transformation to ferrite cementite mixture.
PROCESS: Heating below A1 Holding there for 1 to 2 hours Cooling to room temperature in air TYPES OF TEMPERING: Low temperature tempering (100-200): Martensite- Low C martensite + epsilon Carbide Due to epsilon carbide, it etches rapidly by etchant and appears dark so also called black martensite Hardness reduces up to 60 HRC Brittleness of martensite also decreases.