Why must martensite be tempered

The strength and hardness is a due to elastic strain within the martensite, which is a result of too many carbon atoms being in the spaces between the iron atoms in the martensite. As the amount of carbon in a steel increases up to about 0. Need help selecting a steel alloy or heat treating process? We offer metallurgy consulting. During the tempering process, the carbon atoms move out of the spaces between the iron atoms in the martensite to form the iron carbide particles.

The strain within the martensite is relieved as the carbon atoms move out from between the iron atoms in the martensite. This results in an improvement in the steel toughness, at the expense of reduced strength. The amount of tempering required depends on the particular application in which the steel will be used. In some cases, toughness is not important, so tempering at a low temperature for a short period of time is acceptable. In cases where very strong and tough steel is required a high carbon steel tempered at a high temperature might be used.

Show More. Views Total views. Actions Shares. No notes for slide. Tempering Prof. The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation. Tempering is a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength.

All hardened steels must be tempered before use. It can not be used as such. Hence it is essential to temper it to make it less brittle. Hence it is hard and brittle. The fig. Hardness of Martensite 7. Tempering 9. Microstructure of Pearlite Microstructure of Martensite The equilibrium phases are ferrite and cementite. Stages of tempering The overlapping changes, which occur when high carbon martensite is tempered, are divided into four stages.

Effect of tempering temperature on the properties of eutectoid steel. Figure This is known as secondary hardening. During machining, the temperature of the tool tip increases causing softening in other steels. But High Speed Steels retain their hardness for a longer time due to secondary hardening and therefore can be used for longer time. In steels containing one alloying addition, cementite forms first and the alloy diffuses to it.

When sufficiently enriched, the Fe3C transforms to an alloy carbide. The trapping of carbon inside the martensite adds a further J mol -1 , which makes the total stored energy in excess of J mol -1! Figure 1: The free energy due to the trapping of carbon in martensite, as a function of its carbon concentration.

The results are for a temperature of K. The virgin microstructure obtained immediately after quenching from austenite consists of plates or laths of martensite which is supersaturated with carbon.

In the vast majority of steels, the martensite contains a substantial density of dislocations which are generated during the imperfect accommodation of the shape change accompanying the transformation. The plates may be separated by thin films of retained austenite, the amount of untransformed austenite becoming larger as the martensite-start temperature M S is reduced.

Carbon is an interstitial atom in ferritic iron, primarily occupying the octahedral interstices. There are three such interstices per iron atom. At a typical concentration of 0. Furthermore, there is a strong repulsion between carbon atoms in nearest neighbour sites. This means that carbon atoms almost always have an adjacent interstitial site vacant, leading to a very high diffusion coefficient when compared with the diffusion of substitutional solutes.

In the latter case, the substitutional vacancy concentration is only 10 -6 at temperatures close to melting, and many orders of magnitude less at the sort of temperatures where martensite is tempered. It follows that carbon diffuses much faster than substitutional atoms including iron , as illustrated below. Given that carbon is able to migrate in martensite even at ambient temperature, it is likely that some of it redistributes, for example by migrating to defects, or by rearranging in the lattice such that the overall free energy is minimised.

Indeed, most of the iron carbides can precipitate at low temperatures, well below those associated with the motion of substitutional solutes. This is because they grow by a displacive mechanism which does not require the redistribution of substitutional atoms including iron ; carbon naturally has to partition. This corresponds to a process known as paraequilibrium transformation in which the iron to substitutional solute ratio is maintained constant but subject to that constraint, the carbon achieves a uniform chemical potential.

Martensite is said to be supersaturated with carbon when the concentration exceeds its equilibrium solubility with respect to another phase. However, the equilibrium solubility depends on the phase. Some 0. Although most textbooks will begin a discussion of tempering with this first stage of tempering, involving the redistribution of carbon and precipitation of transition carbides, cementite can precipitate directly.

This is particularly the case when the defect density is large. Trapped carbon atoms will not precipitate as transition carbides but cementite is more stable than trapped carbon. Tempering at higher temperatures, in the range o C for 1 h induces the retained austenite to decompose into a mixture of cementite and ferrite.

When the austenite is present as a film, the cementite also precipitates as a continuous array of particles which have the appearance of a film.

Dark field transmission electron micrograph of martensite in a Fe-4Mo Only the cementite is illuminated. The film of cementite at the martensite plate boundaries is due to the decomposition of retained austenite.

Tempering at even higher temperatures leads to a coarsening of the cementite particles, with those located at the plate boundaries growing at the expense of the intra-plate particles. The dislocation structure tends to recover, the extent depending on the chemical composition. The recovery is less marked in steels containing alloying elements such as molybdenum and chromium. Bright field transmission electron micrograph of martensite in a Fe-4Mo The recovery of the dislocation structure and the migration of dislocation-cell and martensite boundaries leads not only to a coarsening of the plates, but also an increase in the crystallographic misorientation between adjacent plates, as illustrated in the adjacent figure.

The data are from Suresh et al. However, all of these carbides require the long-range diffusion of substitutional atoms. They can only precipitate when the combination of time and temperature is sufficient to allow this diffusion. The figure on the left shows the calculated diffusion distance in ferrite for a tempering time of 1 h. It is evident that the precipitation of alloy carbides is impossible below about o C for a typical tempering time of 1 h; the diffusion distance is then just perceptible at about 10 nm.

The alloy carbides grow at the expense of the less stable cementite. If the concentration of strong carbide forming elements such as Mo, Cr, Ti, V, Nb is large then all of the carbon can be accommodated in the alloy carbide, thereby completely eliminating the cementite. The bright field transmission electron micrograph is of a sample tempered for h, whereas the dark-field image shows a sample tempered for h. The precipitates are needles of Mo 2 C particles. This transmission electron micrograph shows large cementite particles and a recovered dislocation substructure.

There are sub-grain boundaries due to polygonisation and otherwise clean ferrite almost free from dislocations. The plate microstructure is coarsened but nevertheless retained because the carbides are located at plate boundaries. An alloy such as this, containing a large fraction of carbides is extremely resistant to tempering. The original microstructure was bainitic, but similar results would be expected for martensite.

The optical micrograph shows some very large spherodised cementite particles. The ferrite has completely recrystallised into equiaxed grains. Whereas the plain carbon steel shows a monotonic decrease in hardness as a function of tempering temperature, molybdenum in this case leads to an increase in hardness once there is sufficient atomic mobility to precipitate Mo 2 C. Secondary hardening is usually identified with the tempering of martensite in steels containing strong carbide forming elements like Cr, V, Mo and Nb.

The formation of these alloy carbides necessitates the long--range diffusion of substitutional atoms and their precipitation is consequently sluggish. Carbides like cementite therefore have a kinetic advantage even though they may be metastable. Tempering at first causes a decrease in hardness as cementite precipitates at the expense of carbon in solid solution, but the hardness begins to increase again as the alloy carbides form. Hence the term secondary hardening. Coarsening eventually causes a decrease in hardness at high tempering temperatures or long times, so that the net hardness versus time curve shows a secondary hardening peak.

Typical time scales associated with the variety of processes that occur during tempering.