Dear Dr. Mohammad Shahrear Kabir,

carbide precipitation follows a specific order. Primary carbides (M7C3) tended to be generated from liquid steel when the solid fraction reached 84 mol.%. Secondary carbides (M7C3) precipitated from austenite and can hardly transformed into M23C6 carbides with decreasing temperature in air.

Primary carbides hardly changed once they were generated, whereas secondary carbides were sensitive to heat treatment and thermal deformation. Carbide precipitation had a significant effect on steel-matrix phase transitions.

For more details, I suggest you to have a look at the following thesis / papers:

-Microstructure Evaluation and Wear-Resistant Properties of Ti-alloyedHypereutectic High Chromium Cast Iron

Doctoral Dissertation by Qiang Liu - Royal Institute of Technology – Stockholm (Sweden) (2013)

Available at: https://www.diva-portal.org/smash/get/diva2:647897/FULLTEXT01.pdf

-Dynamic Precipitation Behavior of Secondary M7C3 Carbidesi n Ti-alloyed High Chromium Cast Iron

Qiang LIU, Hiroyuki SHIBATA, Peter HEDSTRÖM, Pär Göran JÖNSSON and Keiji NAKAJIMA

ISIJ International, Vol. 53, No. 7, pp. 1237–12 (2013)

Available at: https://www.jstage.jst.go.jp/article/isijinternational/53/7/53_1237/_pdf

-Growth characteristics of primary M7C3 carbide in hypereutectic Fe-Cr-C alloy

Sha Liu, Yefei Zhou, Xiaolei Xing, Jibo Wang, Xuejun Ren & Qingxiang Yang

Scientific Reports volume 6, Article number: 32941 (2016)

Available on RG at: https://www.researchgate.net/publication/307902641_Growth_characteristics_of_primary_M7C3_carbide_in_hypereutectic_Fe-Cr-C_alloy

Good work and my best regards, Pierluigi Traverso.

Dear Dr. Mohammad Shahrear Kabir,

carbide precipitation follows a specific order. Primary carbides (M7C3) tended to be generated from liquid steel when the solid fraction reached 84 mol.%. Secondary carbides (M7C3) precipitated from austenite and can hardly transformed into M23C6 carbides with decreasing temperature in air.

Primary carbides hardly changed once they were generated, whereas secondary carbides were sensitive to heat treatment and thermal deformation. Carbide precipitation had a significant effect on steel-matrix phase transitions.

For more details, I suggest you to have a look at the following thesis / papers:

-Microstructure Evaluation and Wear-Resistant Properties of Ti-alloyedHypereutectic High Chromium Cast Iron

Doctoral Dissertation by Qiang Liu - Royal Institute of Technology – Stockholm (Sweden) (2013)

Available at: https://www.diva-portal.org/smash/get/diva2:647897/FULLTEXT01.pdf

-Dynamic Precipitation Behavior of Secondary M7C3 Carbidesi n Ti-alloyed High Chromium Cast Iron

Qiang LIU, Hiroyuki SHIBATA, Peter HEDSTRÖM, Pär Göran JÖNSSON and Keiji NAKAJIMA

ISIJ International, Vol. 53, No. 7, pp. 1237–12 (2013)

Available at: https://www.jstage.jst.go.jp/article/isijinternational/53/7/53_1237/_pdf

-Growth characteristics of primary M7C3 carbide in hypereutectic Fe-Cr-C alloy

Sha Liu, Yefei Zhou, Xiaolei Xing, Jibo Wang, Xuejun Ren & Qingxiang Yang

Scientific Reports volume 6, Article number: 32941 (2016)

Available on RG at: https://www.researchgate.net/publication/307902641_Growth_characteristics_of_primary_M7C3_carbide_in_hypereutectic_Fe-Cr-C_alloy

Good work and my best regards, Pierluigi Traverso.

Dear Dr. Mohammad Shahrear Kabir,

Agreed with Pierluigi Traverso.

Also, The primary M7C3 is usually larger in size, while The secondary is smaller in size.

You can read this thesis if you need more information about it.

https://www.diva-portal.org/smash/get/diva2:647897/FULLTEXT01.pdf

Here is another link for the differences between primary and secondary M7C3 Carbides.

https://worldwidescience.org/topicpages/s/secondary+carbide+precipitation.html

Good luck

The treated structure is a distribution of carbides in tempered martensit

particles delineated: massive white particles and small black round par

The massive carbide was identified using X-ray microanalysis a

chemical composition was (Fe28, Cr37, V1, Mo1)C33. It was called a prim

form of plates aligned in the direction of machining with an average le

average width of 5µm identified as M7C3 type in accordance with the 1

Fe-C diagram. The chemical composition of submicron round particles

determined with certainty.

Secondary carbides or small black round particles in the form o

average diameter Фmoy=5,3μm presumably of the M23C6 type in accord

binary Fe-C at 12% Cr cut.

Article Sub-zero treatments of AISI D2 steel: Part I. Microstructure...

Article Study of Carbide Evolution During Thermo-Mechanical Processi...

Article Fatigue of cold-work tool steels: Effect of heat treatment a...

Dear Kabir,

Differences are there in growth characteristics.

Some primary M7C3 carbides are formed by layers of shell or/and consist of multiple parts. Primary carbides (M7C3) tended to be generated from liquid steel when the solid fraction reached 84 mol.%. Secondary carbides (M7C3) precipitated from austenite and can hardly transform into M23C6 carbides with decreasing temperature in air. Primary carbides hardly changed once they are generated, whereas secondary carbides are sensitive to heat treatment and thermal deformation. Carbide precipitation had a certain effect on steel-matrix phase transitions. The segregation ability of carbon in liquid steel may be 4.6 times greater that of chromium. Couples of methods for controlling primary carbides are proposed earlier by many researchers time to time.

Analysis of published high-temperature mechanical property data suggests that secondary carbide precipitation might occur during service life causing substantial strengthening of the as-heat-treated material. By microstrucrual analysis using OM, SEM, CLM and TEM, It is found that the primary M7C3 carbide in hypereutectic Fe-Cr-C alloy is irregular polygonal shape with several hollows in the center and gaps on the edge. Some primary M7C3 carbides are formed by layers of shell or/and consist of multiple parts. The electron backscattered diffraction (EBSD) maps confirms that the primary M7C3 carbide consists of multiple parts. The semi-molten M7C3 carbide contains unmelted shell and several small-scale carbides inside, which further proves that the primary M7C3 carbide is not an overall block. Vanadium addition does change the characteristics of the primary and secondary with structure differences.

Ashish

Primary M7C3 comes from liquid usually coarse and it is not easy to dissolved by ordinary annealing heat treatment.

Graphitizing annealing at high temperature( 920-940 oC) for long time more than 4 hr could partially or completely dissolve Primary M7C3 to graphite + element (Fe or Cr...) followed by controlled slow cooling rate.