Question

Compare the martensite and tempered martensite in terms of the phases they contain, ductility, strength and microstructure, explain the reasons for the differences.

Answer 1

COMPARISON OF MARTENSITE AND TEMPERED MARTENSITE:

Comparison of unhampered martensite and tamperd martensite is as given below on the basis of above mentioned requirements. With the help of explaination reason is also cited together with the explaination:

Martensite is very hard in nature and is a form of steel crystal structure. It is formed by diffusion less transformation.

COMPARISON OF MARTENSITE AND TEMPERED MARTENSITE IN TERMS OF PHASE:

Martensite generally consists of body centered tetravonal crystal structure by the rapid cooling of the austinites form of the steels.Very less activation energy is required for the growth of martensite phase as we have earlier told martensite process is diffusion less transformation because of which atoms positions are rearranged rapidly and this can take place at cryogenic temperature itself.

Generally martensite are not described by phase they are described by the microstructure properly.

Tempering is a heat treatment process which is done to martensite in steels. The metal is kept at the temperature at which austinites can not form so thay it properly describe the microstructure, mechanical property and phase of martensite. Due to tempering of martensite, tempered martensite phase change takes place in stages as in very first stage the carbon which is avaialble in excess in the solid solution is broken in to the defects and later forms the cluster inside the solution of solid. After that presipitaion takes place because of which the it can be transformed in to cementite in case of low carbon stell or on case of high carbon allowing elements it can form transition iron carbide.

Furthur heating process leads to the second stage in which all the carbon which was present in excess got precipitated and all the carbide available is converted to cementite which is more stable. If any autenite is also left inside the structure, it will also be decomposed during this stage.

Continuing the tempering furthur coarsenes the carbide present and huge recovery of dislocation structure takes place and result to this recrystallization of the ferrite matrix is processed to grains of equiaxed shape.

So with the tempering of martensite, the internal structure is transformed and properties are improved as explained above in terms of phase

COMPARISON OF MARTENSITE AND TEMPERED MARTENSITE IN TERMS OF DUCTILITY:

Tempering in general terms is the process which is used to increase the toughness by decreasing the hardness of materials. This process of achieving higher toughness also results from increase in ductility. So therefore it decreases the brittleness.

Generally after the quenching process, tempering is performed. In tempering the temperature is properly controlled and quenched part is heated below the lower critical temperature. Heating is done below this temperature so that the martensite hard and quenched microstructure is not destroyed.

The comparison of martensite and tempered martensite in terms of ductility can be shown as martensite in general form is very hard, brittle and having very high internal stress. So the ductility of the martensite in normal terms is very less. Tempering is the process which is used to increase the toughness and improving ductility by releasing internal stress so we can say for tempered martensite the ductility is high.

But for temeleed martensite, ductility achieved by heating can vary depending on the temperature kept during the heat treatment as if the tempering of the quenched part is done in range of 60 to 150 degree, it will not have much effect and very slight internal stresses will be releived and brittleness will also be decrease with slight increase in ductility. After this if the tempering is done in the range of 150 to 210 degree temperature, hardness will be slightly reduced but the internal stress will be relieved in a higher note making the martensite more ductile. But if the temperature during the process is kept above 260 degree, it will result in decrease in ductility and brittleness will be increase.

So as we can see the tempering can give befy good results but the temperature used in the process should be chosen effectively.

COMPARISON OF MARTENSITE AND TEMPERED MARTENSITE IN TERMS OF STENGTH:

Strength of tempered martensite is totally dependent on the degree of tempering as how the tempering is carried out as we have shown in ductility comparison time that if the temperature during tempering is high it can cause increase in brittleness so the strength of the tempered martensite is higher than the normal martensite but the temperature ar which the process is carried out is very important to consider and to perform the proper action.

In general martensite are can not be used in most of the process as due to high hardnes brittleness and internal stresses but as the tempering  is Carried out, it increase ductility, toughness and also relieves the internal stresses. If the temperature is properly defined and process is Carried out, tempered martensite have higher strength.

So as a conclusion the strength of tempered martensite is higher than the martensite but with the proper tempering temperature is the tempering temperature is not selected properly it can reduce the strength of tempered martensite.

COMPARISON OF MARTENSITE AND TEMPERED MARTENSITE IN TERMS OF MICROSTRUCTURE:

T​​​​here is a difference between the microstructure of unhampered and tempered martensite which can be given as in untapped martensite or normal martensite, needle type of microstructure is avaialble while in case of tempered martensite carbide starts forming and the microstructure turns to bushy type.Microstructure improves if the tempering is done properly.

Tempered martensite have extremely fine grained microstructure, they have well dispersed clementine grains in ferrite matrix. Actually martensite in normal form is very hard to use as it is very hard ,brittle and having higher internal stresses so after doing the tempering all these bad properties are modified and a good microstructure is obtained.