How the Crystal Structure of Carbon-Steel Changes during Tempering

This article is a revision containing my contributions from a laboratory assignment in Mechanical Behaviors of Materials. If you are interested in learning more about this study, please feel free to email me at the Contact Form. Paragraphs with (*) are important to the article, but may have not been explicitly authored by me.

Lab Theory and Important Elements

Heat treatment (tempering) is a metallurgical procedure to enhance the strength and toughness of steel. Tempering requires high temperatures, up to 1000℃, that can be reached with a heat treat oven or kiln. Untempered steel is a body-centered cubic (BCC) martensitic structure. The martensite structure exhibits high strength, but is prone to dislocations and line defects due to the tetragonal crystal structure from interstitial carbon atoms. The body-centered tetragonal crystal structure martensite is formed from austenite (ɣ), a FCC structure. During tempering, the tetragonality of martensite transforms to cubic ferrite as carbon is precipitated from the martensite. These carbons precipitate out as carbides, which limit the motion and density of dislocations in the material (Gensamer et al., 2012). 

(*) Quenching is a rapid cooling process in water or oil to obtain a certain material property that prevents undesired low-temperature processes, such as phase transformations from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable and kinetically accessible. In our case, this induces a Martensite transformation where the steel must be rapidly cooled through its eutectoid point such that the Austenite to be metastable. (*)

In order to achieve high strength and toughness, changes in temperature from quenching (rapid cooling) and heating are balanced to combine material properties from different phases. This process is largely dependent on temperature, but it should be noted that the same phase transitions can be achieved using lower temperatures with the trade off of time. When a specimen is cooled past the Martensite boundary (MS) the phases from tempering are locked into the composition.

Other important compositions to this lab are Bainite, a combination of ferrite and cementite, and Pearlite, a product of the transformation from austenite to ferrite and cementite (Callister, 2020). The two are less brittle than martensite, but weaker in strength compared to martensite (Mazilkin et al., 2008).

Both steel specimens, A36 and 1045, can be classified as hypoeutectoid, since their carbon content is below 0.76%. Therefore, the region of interest in the phase diagram is to the far left of fig. 5. For A36, the carbon content is .25-.29 wt%, while 1045 is .45%-.50 wt%. In the case of A36 steel we used a TTT diagram for eutectoid steel, as one was not readily available for A36 (Fig. 2). Using the diagram for eutectoid steel, the A36 Steel begins as ferrite. When heated it becomes austenite (Fig 2, A). When cooled to room temperature, A36 transitions to martensite. After bead blasting, the specimen is placed in the oven at 400°C, where it becomes 50% Austenite, 50% Bainite. After staying in the oven for one hour, the remaining Austenite is transformed to 100% Bainite, and this is the final composition of the material.

 Fig 5 Phase Transformation Diagram

For the 1045 steel, the composition begins as ferrite. Upon heating to 800℃, the martensite undergoes a phase transformation into bainite. The bainite is then quenched to room temperature, which remains as bainite (Fig.3, B). While the specimen is bead blasted, we assumed a time of roughly 1.5 minutes at room temperature before placement in the lower oven.

The specimen is then heated to 400℃ in the lower oven, where the specimen consists of 50% ferrite and 50% martensite (Fig. 3, D). The specimen is held in the oven for one hour, and finally cooled to room temperature where the final composition is 50% pearlite and 50% martensite (Fig.3, F, G).

From this analysis of the phase transformation for A36, which has a final composition of  100% bainite, and 1045 steel, which has a final composition of 50% pearlite and 50% martensite, we would assume the following characteristics for the material which will be tested in Chapter 2.

  1. A36 should prove to have higher toughness but lower strength, as the composition has shifted from 100% ferrite to 100% bainite, which is interpreted from our theory section in (2). 
  2. 1045 will be stronger and tougher than the A36 specimen, due to the presence of martensite, and tougher due to the presence of pearlite.

References

Callister, W. D., & Rethwisch, D. G. (2020). Materials science and engineering. Hoboken, NJ: Wiley.

Gensamer, M., Pearsall, E.B., Pellini, W.S. et al. The Tensile Properties of Pearlite, Bainite, and Spheroidite. Metallogr. Microstruct. Anal. 1, 171–189 (2012). https://doi.org/10.1007/s13632-012-0027-7

Ismail, N. M., Khatif, N. A. A., Kecik, M. A. K. A., & Shaharudin, M. A. H. (2016). The effect of heat treatment on the hardness and impact properties of medium carbon steel. IOP Conference Series: Materials Science and Engineering, 114, 1–10. https://doi.org/10.1088/1757-899x/114/1/012108

Komvopoulos, K. (2017). Mechanical testing of engineering materials. Cognella. 

Komvopoulos, K. (2021). What happens during steel tempering. Mechanical Behaviors of Materials, 1-11.

Mazilkin, A. A., Straumal, B. B., Protasova, S. G., Dobatkin, S. V., & Baretzky, B. (2008). Structure, phase composition, and microhardness of carbon steels after high-pressure torsion. Journal of Materials Science, 43(11), 3800–3805. https://doi.org/10.1007/s10853-007-2222-5.

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