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Martensite in Steel

  • 1.  Martensite in Steel

    Posted 03-08-2021 11:14
    What is the correct way to call Martensite in steel? Is it un-tempered Martensite? Or martensite only?
    I have limited years of experience in steel alloys processing, but never came across the term, "un tempered martensite" for martensite microstructure,  unless there is presence of  residual martensite in tempered martensite microstructure. Please advice.

    Sanjay Kulkarni

    Materials Engineer

    MSSC

    a group company of MITSUBISHI STEEL MFG. CO., LTD.

     




    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------


  • 2.  RE: Martensite in Steel

    Posted 03-09-2021 01:10
    Hi Sanjay,

    Great question! Untempered martensite is what you get just after quenching. It leaves a very needle-like microstructure that's very hard but also quite brittle. Tempering is then performed at much lower temperatures to make the microstructure more homogenous, shorter, and then also much more ductile. The Charpy impact toughness rises substantially and the strength decreases by quite a bit. In some cases, like knives, swords, and blades, the harder material is better so no quenching is used. Conversely, in most engineering applications you need some toughness (aka the resistance to growth of a crack), so the tradeoff of a moderate reduction in strength is worth it. Summing up:

    Tempered martensite has been heat treated to make it slightly softer but much more ductile (and tougher).
    Untempered martensite is fresh out of quenching, very hard but not very tough.

    Best regards!

    ------------------------------
    Joshua Jackson
    CEO
    US Corrosion Services LLC
    Houston TX
    ------------------------------



  • 3.  RE: Martensite in Steel

    Posted 03-09-2021 08:44
    Hi Joshua,
    Thanks for quick response, so the question remains is should I call it un-tempered martensite or just martensite in the report. As far as I know as per metallurgy both types are martensite since they are not tempered. Which is the most appropriate to address in reports martensite or un- tempered martensite for as quenched microstructures?
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 4.  RE: Martensite in Steel

    Posted 03-09-2021 08:54
    Usualyy, for as-quenced microstructure I use "untemperd martensite" or "tetragonal martensite".
    Regards,

    ------------------------------
    Andrea Talamini Minotto
    Villorba Treviso
    ------------------------------



  • 5.  RE: Martensite in Steel

    Posted 03-10-2021 10:00
    Thanks Andrea. I should  designate un-tempered martensite instead of martensite. Clarifies my confusion.

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 6.  RE: Martensite in Steel

    Posted 03-09-2021 09:27
    Untempered martensite should be the right term in your report to distinguish it from tempered martensitic structure. This will provide clarity in your report, particularly for readers who may not have strong Materials/Metallurgical background. 

    Nnaemeka

    ------------------------------
    Nnaemeka Ugodilinwa
    Process Engineer
    StandardAero
    Winnipeg MB
    2043187632
    ------------------------------



  • 7.  RE: Martensite in Steel

    Posted 03-10-2021 09:43
    I would concur if you have untempered martensite to report it as such.  In the power industries use of CrMo steels and in particular martensitic steels like Grade 91 (Creep Strength Enhanced Ferritic Steel), we find untempered martensite in welds which were not given (or given an incorrect) post-weld heat-treatment and in components which were improperly heat-treated.  We would generally use a combination of hardness, optical metallography, and SEM-EBSD to confirm untempered martensite.

    ------------------------------
    John Shingledecker
    Electric Power Research Institute
    Charlotte NC
    (865) 201-1252
    ------------------------------



  • 8.  RE: Martensite in Steel

    Posted 03-10-2021 10:04
    Thank you very much John. I am following your advice.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 9.  RE: Martensite in Steel

    Posted 03-10-2021 10:02
    Thanks Nnaemeka. I appreciate you taking time in replying my question. I will take your advice.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 10.  RE: Martensite in Steel

    IMS Board Member
    Posted 03-10-2021 13:35
    Sanjay:
    The term that you should use in your report depends on your interpretation and understanding of the true structure. The features observed in a properly prepared metallographic sample will be different for tempered and untempered martensite. So, it is important that you understand the difference between the two structures, how these structures could occur from the history of the steel, and what significance the difference has to your investigation. If a differentiation doesn't really matter for a particular investigation, use of the simpler term martensite would be correct in either case. That is assuming that the structure of the steel is not bainite or very fine pearlite that you are mistaking for martensite.

    I suggest that you review the book, Light Microscopy of Carbon Steels by Leonard E. Samuels, which has great images of different structures and descriptions of how they formed, to inform yourself on interpretation of microstructures and proper terminology. The book is conveniently available for purchase on the ASM web site.

    Good luck and happy etching.

    ------------------------------
    Larry Hanke
    Principal Engineer
    Materials Evaluation And Engineering Inc
    Minneapolis MN
    ------------------------------



  • 11.  RE: Martensite in Steel

    Posted 03-12-2021 09:06
    Thanks Larry. I will refer the book Light Microscopy of Carbon Steels. 
    Side discussion: It is easy to differentiate martensite and bainite phase in un tempered martensite microstructure, but I always struggled to differentiate between tempered martensite and upper transformation products, specially in low carbon boron steels. Being in a business of heat treating, I occasionally get parts those are slack quenched,(I guess slack and delayed quench are the production terms in HT world).  I always relied on microhardness test and  a dark case at the surface of OD after etching with Nitol. Tempered martensite in low carbon steels and UTP on the surface of OD is always challenging for me.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 12.  RE: Martensite in Steel

    Posted 03-10-2021 12:06
    Hi Sanjay,
    When you have to name Martensite it is linked to the heat treatment that the steel has received. If it is as-quenched steel (heated at high tempered and then cooled very fast) the microstructure product is an un-tempered martensite. If the steel is quenched AND tempered, then tempered martensite is produced. Usually steels in industrial applications are used quenched and tempered in order to combine strength and ductility (as some of the colleagues said), then you have tempered martensite.

    This said, maybe you need to know that sometimes during manufacturing, when quenched and tempered steel are grinded to improve the surface finish of the parts, accidents may happen. Some areas of the parts could be overheated and the tempered martensite could locally transforms in "un-tempered martensite" if the reached temperature is above what we call "transformation temperature" (temperature at which martensite transforms in austenite) or "over-tempered martensite" when the temperature reached locally is above the temper temperature. Thus, the resulting microstructure would be "tempered martensite + areas of un-tempered and over-tempered martensite". Hope it is not confusing for you Sanjay.
    Please, do not hesitate if you have further questions.

    Nihad

    ------------------------------
    Nihad Ben Salah
    President
    NBS- M&P Consulting
    Canada
    https://www.nbsmpconsult.com/en/home
    ------------------------------



  • 13.  RE: Martensite in Steel

    Posted 03-12-2021 09:17
    Hello Nihad. 
    The points you have made in you advice is true. I am not confused. I get those issues many times.  It happens in peeling or shearing process. The microstructure at the edge where un tempered martensite gets etched white. In my early days of this job when I was analyzing the first sample of the fracture,  I  was confused with ferrite, but as a proof when I did microhardness on the layer It was Rc 65 which was not ferrite, so I went and found papers which talked about un tempered martensite formation in machining process. Thanks for your advice.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 14.  RE: Martensite in Steel

    Posted 03-13-2021 12:20
    In some high alloy steels, the stress relief that accompanies tempering allows some retained austenite to transform into untempered martensite (also called fresh martensite) upon cooling. For this reason, some high alloy steels (especially tool steels) are double tempered. The cooling after the first temper is important -- just doubling the temper time will not have the same effect

    --
    John Grubb





  • 15.  RE: Martensite in Steel

    Posted 03-15-2021 09:48
    Hello John,
    Thank you very much for the information. I have seen un-tempered martensite (in band form) in hot rolled-normalized condition. Mostly it was in 9254 and some patented alloy spring steels. It was in core areas. Is that because of retained austenite? Please see the screen shot below below,


    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 16.  RE: Martensite in Steel

    Posted 03-15-2021 13:51
    I cannot say what is retained austenite or fresh (untempered) martensite in your micro. My background is more in stainless steels (including some partially transforming martensitic alloys) and tool steels. The effect of untempered martensite depends partly on its carbon content, with low-carbon martensites (possible in high alloy materials) being somewhat forgiving, but never as tough as tempered martensite. 
    In general, retained austenite will be found between the martensite laths. For some materials, this retained austenite may provide increased ductility and toughness.
    Center segregation is common in continuous cast and ingot cast steel. https://www.jstage.jst.go.jp/article/isijinternational/53/6/53_935/_pdf/-char/en is a reasonable review of segregation in cast steels. 

    --
    John Grubb





  • 17.  RE: Martensite in Steel

    HTS Board Member
    Posted 03-16-2021 08:59
    The microstructure you show appears to be plate type martensite in retained austenite which is similar to the higher carbon content microstructures commonly found near the surface in carburized steel components. The presence of the high silicon content in 9254 is known to promote the retained austenite by slowing the formation of cementite precipitates. There are inevitable areas where some alloy and carbon segregation is present at the center of the bar from center solidification during casting. For example, see the data  in Chapter 1 of The Making, Shaping, and Treating of Steel Long Products published by AISTech. where it is shown that the likelihood of finding these small areas varies along the length of the bar. The cooling rate during normalizing will have an influence on the appearance of these areas due to the kinetics of carbon enrichment. These microstructures will not likely form during quenching due to the faster cooling rates preventing the carbon diffusion that causes the localized high carbon regions.

    Robert Cryderman
    Research Associate Professor
    Metallurgical and Materials Engineering
    Colorado School of Mines

    ------------------------------
    Robert Cryderman
    Research Associate Professor
    Colorado School Of Mines
    [Golden] [Colorado]

    ------------------------------



  • 18.  RE: Martensite in Steel

    Posted 03-17-2021 09:17
    Hi Robert,
    Thank you very much for the advice and valuable paper that I can use as a reference documentation for internal discussions with our team.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 19.  RE: Martensite in Steel

    Posted 03-17-2021 10:04
    Hi Sanjay,
    It is very unsual to see un-tempered martensite embedded in the core of normalized steel unless there is a problem of heterogeneity. My only explanation is that at this location the chemistry of the steel may be different (chemical heterogeneities), so is the response to hot rolling (impacting grain size) in a way that stabilizes austenite (lowering Mf and pushing the CCT curve to the right) allowing the formation of martensite + retained austenite upon air cooling (normalizing is usually air cooling) OR formation of a problematic localized Widmanstatten structure (hard and brittle). I am putting forward the Widmanstatten structure because the microstructure shown in the micrograph at the banding area suggests it. All the ingredients are there but in order to validate it you may need more results. You can analyze this area by SEM/EDS to assess if the problem is chemical heterogeneity.
    Throwing ideas here.
    Have a good day,
    Nihad

    ------------------------------
    Nihad Ben Salah
    President
    NBS- M&P Consulting
    Canada
    https://www.nbsmpconsult.com/en/home
    ------------------------------



  • 20.  RE: Martensite in Steel

    Notre Dame Chapter Adm
    Posted 03-09-2021 17:20

    Sanjay:

    Immediately after quenching, martensite is untempered, but untempered martensite is usually undesirable because it is brittle (practically no ductility).  So the steel will be typically tempered to restore some level of ductility (tempered martensite).  In modern Advanced High Strength Steels (AHSS) retained austenite may transform into martensite (the steel "TRIPS") as a result of the application of strain.  This is desirable because the transformation requires the absorption of energy.  Such steels may be used in automotive applications to manage crash energy.

    I hope this helps.

    Nassos






  • 21.  RE: Martensite in Steel

    Posted 03-10-2021 10:18
    Thank you very much Nassos. I greatly appreciate it.
    Thanks to all of you for the advice and sharing some experiences. We are in automotive industry and tier one supplier. There are not many professionals in our company who understands metallurgy.
    I do agree my reports should be understood to all group of people not just the materials professionals. In most of my carrier, I worked with materials professionals so never had this issue.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 22.  RE: Martensite in Steel

    Posted 03-10-2021 14:18
    It is particularly useful to include key variants of untempered martensite, depending on the circumstances. Key variants include: untempered martensite, carbon content (and contents of other interstitial alloy elements), and residual stress. These are all important for failure analyses, life predictions and so on.

    ------------------------------
    Neil Kilpatrick
    Owner
    GenMet LLC
    Winter Springs FL
    (407) 760-7293
    ------------------------------



  • 23.  RE: Martensite in Steel

    Posted 03-12-2021 08:46
    Thanks for your advice Neil. Mostly we use 5160H, 9250 and low carbon boron steels. These grades are cheaper and widely available in the market and have been used for many years, so our customers have confidence in it.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 24.  RE: Martensite in Steel

    Posted 03-11-2021 10:56
    While I can't confirm this, I would expect that George Krauss' book "Steels: Processing, Structure, and Performance" (available in the ASM book store) would be the current authorative referance for identifying and describing the various 'flavors' of martensite.

    ------------------------------
    Bruce Boardman FASM
    Geneseo IL
    (309) 944-8700
    ------------------------------



  • 25.  RE: Martensite in Steel

    Posted 03-12-2021 08:37
    Hello Bruce,
    I will definitely check it out. Thanks a lot for valuable advice.
    Regards,

    ------------------------------
    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
    ------------------------------



  • 26.  RE: Martensite in Steel

    Posted 03-18-2021 01:22
    Hi Sanjay,
    Residual stresses can also influence martensite transformation, and mechanically induced martensite can reverts back to austenite when stress is removed. You can find a brief description of Stress Induced Martensite (SIM) in Physical Metallurgy Principles (p. 556, 4th edition) or in the original article by Schroeder and Wayman, Acta Met, 1979.

    ------------------------------
    Reza Abbaschian
    Distinguished Professor
    University of California Riverside
    Riverside CA
    ------------------------------