Martensitic transformation steel - effect of austenitizing temperature

Martensitic Transformation – Austenitizing temperature effect

Martensitic Transformation is basically considered as most commonly used non-equilibrium cooling transformation. This transformation largely depends upon the heating temperature of the austenitizing temperature of steel. The basic purpose of this experimental work was to examine the effect of different Austenitizing temperature on phase transformation of given plain carbon steel AISI 1045 samples.

The samples were preheated at 680 oC, 780 oC, and 880 oC, holding time of 30 minutes was provided. Afterward the samples were subsequently cooled by water quenching media. Then after metallography the microstructure and hardness of these steels were observed by metallurgical microscope and Brinell hardness tester, respectively. Results have shown that heating above 727 oC, leads to austenitic transformation. The sample heated at 880 oC was the hardest of all showing complete austenitic transformation resultantly generating complete martensitic transformation.

For understanding, Follow TTT Diagram in Steel to learn about the critical cooling rate necessary for this transformation. For achieving this goal, a higher cooling rate necessary may result in various heat treatment defects which must be considered while looking for efficient quenching methods.

Introduction

Anything which is physically distinct and has homogenous composition is called phase. There can be macro phases (composites) and micro phases (ferritic). There are different types of phases based on physical state (solid, liquid, gas) and on uniformity (homogeneous, heterogeneous).

Phase transformation is referred to as the change of one phase into another phase. The transformation in which the composition of phase does not change until the transformation completes called congruent phase transformation. When composition changes during phase transformation, known as incongruent phase transformation (martensitic transformation). In order to understand the negative effect of grain coarsening and carbon concentration, “Study Widmanstatten structure in steel”.

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For martensitic transformation understanding, phases of Iron
Different Phases of IRON

In Fe-C diagram there are different phases as α, δ and γ there is another phase ϵ. α and δ are BCC structure while γ is FCC. ϵ is HCP structure which exists at very high pressure. So, it does not exist on equilibrium Fe-C diagram under equilibrium conditions of temperature and pressure. When a piece of steel of appropriate size is heated to different austenitizing temperature (680oC, 780 oC and 880 oC) which is then water quenched and the phase transformation of 1045 steel can be studied. Heating on different austenitizing temperature gives different mechanical properties i.e., hardness and different microstructures.

Martensitic Structure is common blade steels like W1 steel, 18Cr13MoV steel which is used in kitchen knives and hunter knives. Tool steels like D2 steel are also using martensitic structure due to characteristics like wear resistance and high hardness.

Quenching is a process of rapid cooling of steel from the austenitizing temperature. Quenching results in austenitic to martensitic transformation (which is a non-equilibrium constituent). A medium that is used for quenching is known as quenchant. The effectiveness of the quenching process largely depends upon the characteristic of the quenchant used.

You can further study about Diffusionless transformation on WIKIPEDIA.

Experimental Work

  • Three samples of the same size and composition (AISI 1045) were taken.
  • Hardness values were measured with a Brinell hardness testing machine.
  • Dimensions of each sample were measured.
  • Then three samples were heat-treated at 680oC, 780oC, and 880oC respectively.
  • Samples were held for 30 minutes.
  • After heating in the furnace, all samples were put into the water for quenching.
  • After this, the coarse and fine grinding of samples was done.
  • Then coarse and fine polishing and etching of samples were performed.
  • Microstructures were observed with a metallurgical microscope.
  • For the characterization of the microscope, scaling is important. You can study in Microstructure Scaling Article.
  • Again, hardness values were taken by using the Brinell hardness testing machine.

Observation and Caculations

Sample Heating TemperatureBefore Heating BHNAfter heating BHN
Sample 1680217179
Sample 2780197201.45
Sample 3880179625

Microstructural Study

Sample # 1

High pearlite and low ferrite microstructure
Optical Micrograph at center of hypoeutectoid steel(AISI 1045), heated 680 C, Sample 1, Dark Region depicts pearlite whereas white region depicts ferrite etched with 2% NITAL, Original Magnification 400X

Sample # 2

Pearlite and ferrite microstructure
Optical Micrograph at center of hypoeutectoid steel(AISI 1045), heated 780 C, Sample 2, Dark Region depicts pearlite whereas white region depicts ferrite etched with 2% NITAL, Original Magnification 400X

Sample # 3

Martensitic transformation from Austenitizing temperature, needle like structure depicts martensite
Optical Micrograph at center of hypoeutectoid steel(AISI 1045), heated 880 C, Sample 3, Dark Region depicts martensite whereas white region depicts retained austenite etched with 2% NITAL, Original Magnification 400X

Discussions (Austenitizing Temperature on Martensitic Transformation)

In Micrographs of sample 1 and 2, relative amount of ferrite and pearlite phases is approximately 50% and 50% respectively. On applying lever rule,

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level rule

But the original % age of steel specimens was 0.45 %. The probable reasons could be personal error, poor kinetics of transformation, non-equilibrium heating, etc.

Martensitic transformation and Austenitizing temperature link
Hadness chart of samples

The variance in hardness depicts the phase transformations in steel. In sample 1, hardness decreases after heating at 680 oC because at this temperature no phase transformation takes place in steel. This temperature is below austenitizing temperature. The little decrease in hardness is due to the recovery mechanism due to heating.

In sample 2 hardness increases but the micrographic study depicts equilibrium phases only. So, the reason is that the eutectoid transformation temperature is 727oC and the sample was heated at Austenitizing temperature 780 oC, so there would be a very small amount of transformation, which on quenching converts to a very small amount of martensite, which increases hardness but did not reveal at 400X.

As for Sample 3, heated at Austenitizing temperature – 880 oC, a complete transformation took place i.e. austenite formed completely. So, on water quenching, it converts into martensite as depicted in the micrograph. The extraordinary increase in hardness is due to this martensitic transformation. There is an error in the hardness value for this sample, as the Brinell scale is up to 600 BHN, but there should be an increase in hardness due to martensitic transformation.

Conclusion (Austenitizing Temperature on Martensitic Transformation)

It is concluded that with increasing heating temperature above 727 oC, steel transforms into austenite. On heating at austenitizing temperature, the kinetics of austenitic transformation improves leading to rapid and complete transformation. Austenitic to martensitic transformation leads to a drastic change in hardness values.

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