Why martensite has high strength

Evidence is presented from x-ray diffraction to show that this behaviour is due to the presence of intra-granular stresses that are residues after the shear transformation from austenite to martensite. These internal stresses are reduced in magnitude by plastic deformation and also by tempering. Reduction of internal stress due to plasticity is shown by a decrease in XRD line broadening after deformation.

A simple model is presented in which the stress-strain behaviour is controlled by relaxation of the internal stresses almost up to the point of the ultimate tensile strength. It demonstrates that only a very small fraction of the material remaining in a purely elastic state provides a large stabilising effect resisting necking.

A corollary of this is that the uniform elongation of martensitic steel actually increases with increase in the strength level. Effects of heat treatment are also reproduced in the model, including the increase in conventional yield stress Rp 0.

Martensite is well known to be a very strong phase in steels and has been studied extensively over many decades. In addition to its traditional uses such as in tools and springs, there is renewed interest in it for applications in vehicles because of the weight savings that can be achieved through down-gauging.

Such applications, however, place demands not only on strength but also for some plasticity that is required during forming and for energy absorption in crash situations. This has stimulated considerable activity in recent years into new processing methods as well as into the microstructures of martensitic steels. Interest has centred, in particular, on the lath martensite structures found in low-medium carbon steels. Current understanding and findings have been reviewed by Maki.

Maki: Proc. The tensile strength and conventional yield stress 0. McEvily, R. Ku and T. Johnson: Trans. AIME , , Nakashima, Y. Fujimura, H. Matsubayashi, T. Tsuchiyama and S. Takaki: Proc. Hutchinson, J. Karlsson, D. Lindell, M. Tornberg, F. Lindberg and M. Thuvander: Acta Mater. Krauss and co-workers 2 G. Allain, O.

Bouaziz and M. Zhang, T. Ohmura and K. In principle, both homogeneous long range Type I and inhomogeneous short range Type II internal stresses could be involved.

The possibility that these can account for micro-yielding phenomena in martensite has been clearly demonstrated. Muir, B. Averbach and M. Cohen: Trans ASM , 47 , The present work builds on this last principle and concentrates on the role of Type II residual stresses since the materials are in the form of thin sheets where the Type I stresses are accordingly small. The novelty in this approach lies in demonstrating that the role of the internal stresses is much more persistent than previously has been supposed.

In fact, such stresses play a dominant role in the stress-strain behaviour of martensite both before and beyond the conventional 0. This view will be supported by experimental measurements and incorporated into a very simple model of yielding which is based on the criterion that crystals of martensite yield sequentially, during loading, when the combination of external stress and local residual stress reaches the plastic flow stress of the martensite.

We concentrate here on the local Type II internal stresses that exist between different crystals in the metal, arising from the heterogeneous shearing that is intrinsic to the martensitic transformation. Such stresses can be assessed by x-ray diffraction XRD of line broadening.

There is typically more than one source of line broadening in XRD since dislocations present in the martensite also make a contribution This classical source of line broadening has been reviewed, for example by Warren 15 G. Masing: Wissenschaftliche Veroffentlichungen aus dem Siemens-Konzern , 3 , Experiments have been carried out on three steels with chemical compositions shown in Chemical compositions of the investigated steels in wt. Inserted in Fig. Hall-Williamson plot for as-quenched steel B showing that line broadening arises almost entirely as a result of internal strains.

Inserted peaks for ferrite and martensite are scaled to the same height. Data for four other steels having different carbon contents from a previous study 10 B. Relationship between tensile strength and mean internal stress for four quenched martensitic steels with carbon contents in the range 0.

It may at first seem strange to invoke the tensile strength in a relationship of this nature, rather than, for example, the 0. However, we consider that the tensile strength is actually the best available measure of the stress to cause plastic flow in the martensite as will be justified further below. For the purpose of clarity we point out that the internal stresses are not regarded as being the cause of strength in martensite. On the contrary, it is the intrinsic strength of the martensite that sets a limit to the magnitude of the internal stresses.

The observation that large internal stresses exist in the martensite and the realisation that these must influence the behaviour during loading provided the basis for a model. The principle adopted is similar to that proposed by Masing 17 F. Montheillet and G. Damamme: Adv. Although this cannot be completely correct, it is a reasonable first approximation since the shape change during transformation is dominated by simple shear. Steels with a fully martensitic microstructure are associated with the highest tensile strength — grades with a tensile strength of MPa is commercially available, and higher strength levels are under development.

Figure 1: Schematic of a martensitic steel microstructure. Ferrite and bainite may also be found in small amounts. To create MS steels, the austenite that exists during hot-rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line.

Adding carbon to MS steels increases hardenability and strengthens the martensite. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various combinations to increase hardenability.

These steels are often subjected to post-quench tempering to improve ductility, so that extremely high strength levels can be achieved along with adequate ductility for certain forming processes like Roll Forming.

Engineering and true stress-strain curves for MS steel grades are presented in Figures 4 and 5. Figure 4: Engineering stress-strain curves for a series of MS steel grades. S-5 Sheet thicknesses: 1. Figure 5: True stress-strain curves for a series of MS steel grades. In addition to being produced directly at the steel mill, a martensitic microstructure also can be developed during the hot stamping of press hardening steels. Examples of current production grades of martensitic steels and typical automotive applications include:.

Some of the specifications describing uncoated cold rolled 1st Generation martensite steel MS are included below, with the grades typically listed in order of increasing minimum tensile strength. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements. Finally, the relatively low yield strength of the fresh martensite, significantly lower than for the tempered conditions, is discussed in relation to the two available theories.

This harsh application environment puts significant requirements on the performance of steels used for the tipper bodies. The steels need to have a high resistance towards dent formation and abrasion to increase the lifetime of the product.

During tempering, the initial martensitic microstructure will evolve towards the equilibrium state of ferrite and carbides. The microstructural evolution can be divided into the following concurrent processes: i recovery with dislocation annihilation and residual stress relief[ 12 , 13 ]; ii precipitation of various carbides heterogeneously at defects like boundaries and dislocations[ 14 , 15 , 16 ]; iii grain growth.

Six possible carbide phases may precipitate in this alloy system depending on alloying content and heat treatment conditions. The present work aims at exploring the microstructure—property relationship of wear-resistant CrMoV steels. The mechanical behavior measured by tensile testing of a low-alloy CrMoV martensitic steel in quenched and tempered conditions is correlated with the quantitative microstructural evolution. Modeling of carbide precipitation supports the experimental characterization to understand the microstructural evolution.

Finally, yield strength modeling attempts to elucidate the contribution of different microstructural parameters to the yield strength. The investigated material is a commercial alloy with the nominal composition Fe The acquisition time for each step was 6 seconds.

A standard Al 2 O 3 powder sample was used to measure the instrumental peak breadth. The peak broadening of cubic crystals is a combined effect of lattice strain broadening and particle size broadening.

The analysis procedure presented in Reference 25 , 26 , through 27 was adopted. Vickers hardness measurements using a load of g were conducted in a MXT-a1 microhardness tester.

Ten tests were performed on each sample. The average hardness and standard deviation were subsequently calculated. Thereafter, three tensile test specimens were cut from the middle part of the heat-treated samples by wire electrical discharge machining. The specimens had a rectangular cross section and a dog-bone shape.

Gauge length and width were 32 and 8 mm, respectively, and the thickness was 2. Three tensile tests per condition were performed at ambient temperature in a universal tensile testing machine with a kN capacity at a constant crosshead speed of 0. Tensile elongations were determined using a clip-on extensometer with a measurement distance of 25 mm.

The interfacial energy between precipitate and matrix is evaluated utilizing the generalized nearest-neighbor broken-bond GBB model[ 34 ] and the interface curvature effect is considered by the size-correction model,[ 35 ] as implemented in MatCalc. Many studies[ 9 , 10 , 36 , 37 , 38 , 39 , 40 ] have been carried out trying to determine the effective grain size that determines the strength of lath martensite, however, without a consistent picture.

It is widely accepted that high-angle grain boundaries HAGB above 15 deg are effective obstacles for slip. Thus, in the present work, the effective grain size of lath martensite was evaluated by the line intercept method with a threshold misorientation of 15 deg.

The results indicate that the effective grain sizes of the investigated three samples are constant within the experimental error margin, i. It should be noted that the effective grain size of martensite evaluated here is close to the block width, as blocks are the minimum units of martensite with boundaries above 15 deg, whereas sub-block boundary misorientations are about 10 deg and lath boundaries about 2 to 3 deg.

This is in agreement with the previous reports[ 10 , 14 ] stating that the size of blocks remains constant during tempering. Thus, grain coarsening has a negligible effect on the evolution of strength during tempering in the present work. The XRD results for the as-quenched and tempered samples are shown in Figure 2 a. Six bcc iron peaks for each diffraction pattern suggest a single martensite phase. Thereafter, peak width remains almost constant for prolonged tempering up to 5 hours.

The evolution of peak broadening of six diffraction peaks vs tempering time at tempering is shown in Figure 2 c and it is seen that the peak broadening drops drastically at the early stage of tempering 5 minutes followed by a slow gradual decrease. The dislocation density of the as-quenched condition is 2.

The dislocation density further gradually decreases to 1. The evolution with decreasing dislocation density is slower at the lower tempering temperature as seen in Figure 2 d. The dislocation densities of all samples are nonetheless high, in the order of 10 This high dislocation density is important for the properties of the steel, affecting both strength and ductility.

The transition from metastable M 3 C to stable carbides is also captured by the modeling. The volume fraction plots see Figure 3 b indicate that the M 3 C starts to dissolve before 10 hours and after about hours M 7 C 3 starts to dominate. The simulation of carbides precipitation in a multi-component system Fe The results from the modeling of cementite precipitation at both tempering temperatures are presented in Figure 4 for comparison. The difference between the two temperatures is that the increase of cementite mean radius, volume fraction, and number density is shifted towards longer tempering times due to the lower diffusivity of carbon at the lower temperature.

The true stress—strain curves, work hardening rate, yield strength 0. It is noted that the onset of plastic deformation for the as-quenched sample is earlier Rp 0. After the early yielding of the as-quenched samples they experience a significant work hardening until the UTS at MPa, i. The early yielding is typical for single-phase fresh lath martensite, and several mechanisms have been proposed to explain this behavior. One explanation is based on the non-uniform distribution of carbon.

This kind of carbon dispersion also occurs during tempering of martensite and it is not clear why tempered martensite would not experience early yielding in such case.

These dislocations are easily moved by applying a low stress, resulting in an early plastic deformation. Another explanation is based on the residual stresses in the fresh martensitic microstructure also created during the martensitic transformation, where the plastic flow initiates firstly in the matrix where the local residual stress is aligned to add to the applied stress.

It is, hence, quite possible that both mechanisms are acting and so far there is no experimental evidence for either mechanism being more significant. The early yielding behavior of the fresh martensite makes it difficult to model the yield strength of both fresh and tempered martensite using the same set of semi-empirical strength models, though it is possible to get agreement with experiments using more sophisticated treatments such as crystal plasticity modeling.

After tempering, the effects of mobile dislocations and micro-stresses are no longer prominent. Most of the remaining dislocations should be pinned by each other and by carbon segregation as Cottrell atmospheres[ 43 ] and by cementite precipitates.

These pinned dislocations in the tempered samples need larger stress to move, so the tempered samples have relatively higher yield strength. The work hardening rate of the as-quenched sample is significantly higher than those of the tempered samples see Figures 5 b and d. There are two explanations for this phenomenon: i rapid dislocation multiplication caused by deformation[ 47 ] and ii sequential yielding of regions with different strength levels.

This may be due to dislocation annihilation usually called dynamic recovery[ 48 ] or the concurrent plastic deformation of all the regions.

The stable mechanical properties of the tempered samples plausibly result from the decrease of dislocation strengthening due to dislocation annihilation, and reduced solid solution strengthening by carbon which are counteracted by the precipitation strengthening when nanoscale cementite precipitation occurs.

This could be due to the larger recovery at the higher temperature with less significant residual stresses and fewer dislocations, causing less stress concentration and thus higher ductility. The Vickers hardness vs tempering time at the two tempering temperatures is presented in Figure 6. The results show a similar trend at the two temperatures. The hardness first sharply decreases at the early stage of tempering, before becoming stable for a fairly long tempering time, and finally it slightly decreases.

This first decreasing stage is caused by the recovery process during tempering, i. However, thereafter when a high number of cementite particles have formed and grown see Figure 4 it seems to counteract the strength decrease due to further dislocation annihilation and less solid solution hardening from carbon. It should be noted that these results are non-conclusive, but give an indication that the effect of cementite precipitates should be considered to accurately predict hardness and strength of martensite tempered at a fairly low temperature, since nanoscale cementite can also provide a significant strengthening effect.

This is consistent with the aforementioned strength results. Overall, the evolution of dislocation density and cementite precipitation correlated with the carbon in solid solution are the main factors controlling the mechanical property evolution of martensite in this work. Furthermore, since most carbon exist in the form of cementite and the mean radius and volume fraction of cementite are stable during extended tempering, it is reasonable to presume that carbon depletion in the matrix and precipitation play a negligible role in the evolution of strength at this stage.

To analyze the microstructural effects on the yield strength, the individual microstructural contributions to the yield strength are modeled. This contribution is about MPa following the Hall—Petch equation[ 39 , 52 , 53 ]:. Secondly, the carbon content in solid solution is evaluated by considering the nominal composition and subtracting the carbon bound to the cementite precipitates at each tempering condition. The substitutional elements are considered constant in the matrix.

Thereafter, solid solution strengthening is evaluated to be to MPa depending on the tempering condition following Eq. Thirdly, dislocation strengthening provides a major contribution to the high strength of the steel within a range of to MPa using the fitted results for the dislocation density from the XRD measurements Figure 2 d and by the following equation[ 36 ]:.

Fourthly, the precipitation strengthening is evaluated considering the Orowan mechanism, as cementite is usually incoherent with the Fe bcc matrix. Precipitate mean radius and volume fraction come from the precipitation modeling Figure 4. The large volume fraction of cementite with nanoscale size has an evaluated precipitation strengthening of maximum MPa, which is in agreement with prior work.

All the individual strength contributions are summarized in Table III and the corresponding microstructural parameters are summarized in Table IV. The tested yield strength Rp 0. This is expected considering the early plastic yielding of fresh martensite as discussed earlier.

It should be noted that the presence of residual stresses contributes to the high UTS of the as-quenched sample, but decreases the yield strength Rp 0.

Modeling of yield strength through adding the individual strengthening mechanisms in the bar charts, compared to the experimental results indicated by stars. In the modeling of the yield strength Rp 0. The modeling of precipitation using LSKW approach, calling CALPHAD thermodynamic and kinetic databases, is in quite good agreement with the experimental results considering the precipitating phases and their quantities as well as transition between them.

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