1) Martensite is the hardest and most brittle microstructure obtainable in a given steel.
2) Martensite hardness of the steel is a function of the carbon content in that steel.
3) Martensite results from cooling from austenitic temperatures rapidly by pulling the heat out using a liquid quenchant before pearlite can form.
4) As quenched Martensitic structures are too brittle for economic use-they must be tempered.
5) Reheating as quenched Martensite to a temperature just below the AC1 results in the best combinations of strength and toughness.

This is what you get when you cool faster than the critical cooling (pearlite transition) rate- Martensite

Hardness of martensite is a function of carbon content

Softening of martensite in 0.35%C, 0.8% C, and 1.2% C carbon steels by tempering at the indicated temperature for 1 hour.

Because Martensite transformation is almost instantaneous, the Martensite has the identical composition of the parent phase, unlike ferrite and pearlite which result  from a slower chemical diffusion process, so each have different chemical compositions than the parent austenite.
Formation of Martensite involves a transformation from a body-centered cubic structure to  body-centered tetragonal structure. The large increase in volume that results  creates a highly stressed structure. This is why Martensite has a higher hardness than Austenite for the exact same chemistry…
Photo  and Graphs Credit: Cold Finished Steel Bar Handbook

The role of Manganese in steel in our precision machining shops.

Manganese ore like this comes from Turkey.

Carbon is a chemical element that is the primary hardening constituent in steel. Manganese is a chemical element that is present in all commercial steels, and contributes substantially to a steel’s strength and hardness, but to a lesser extent than does carbon.

  1. The effectiveness of Manganese in increasing mechanical properties depends on and is proportional to the carbon content of the steel.
  2. Manganese also plays an important role in decreasing the critical cooling rate during hardening. This means that manganese helps to increase the steel’s hardenability. It’s effect on hardenability is greater than that of any of the other commonly used alloying elements.
  3. Manganese is also an active deoxidizer, and is less likely to segregate than other elements.
  4. Manganese improves machinability, by combining with sulfur to form an soft inclusion in the steel that promotes a steady built up edge and a place for the chip to break.
  5. Manganese improves yield  at the steel mill by combining with the sulfur in the steel, minimizing the formation of iron pyrite (iron sulfide) which can cause the steel to crack and tear during high temperature rolling.

Manganese is an important constituent of today’s steels.
Now you know a few reasons why Mn (the abbreviation for Manganese) is the second element shown on the chemical analysis report (right after carbon).
It’s That Important!
Mn Ore Photocredit.

The weldability of steels is influenced primarily by the carbon content. At higher carbon levels, steels may need either pre- or post- weld heat treatment in order to prevent stress build up and weld cracking.
Generally speaking, if the Carbon Equivalent (CE) is 0.35 or below, no pre- or post- weld  thermal treatment  is needed. In our experience with maintenance welding, we have found that preheating was beneficial between 0.35 and 0.55 CE. Above 0.55 CE we usually both pre- and post- weld heated to relieve stress and prevent cracking.
So CE= .35 max.
However the other elements that are contained in the steel also have an effect on the steel’s “carbon equivalence.” These additional elements can really add up in scrap fed electric arc  furnace steels that now predominate in our market.

Electric Arc Furnaces are predominately scrap fed.

Photo credit.
Here are two formulas for calculating Carbon Equivalents.
CE=%C+(%Mn/6)+(%Cr+%Mo+%Va)/5 + (%Si+%Ni+%Cu)/15
This is the first formula I learned when I took over metallurgical support for  maintenance ‘back in the day.’
In this formula you can see that 6 points of Manganese are approximately equal to one point of Carbon.  5 points of Chrome, Moly or Vanadium are roughly equal to a point of Carbon, while it takes about 15 points of Silicon, Nickel or Copper to get about the same effect as one point of Carbon.
The GE formula for Carbon Equivalency is CE= C+(Mn/6)+(Ni/20)+(Cr/10)+(Cu/40)+(Mo/50)+(Va/10). If this is less than .35 max, you should have no need to pre or post weld thermal treat in most cases.
As long as CE is no more than .35, you probably won’t need to preheat or post weld stress relieve your welded parts. above .35 CE, you may need either or both depending on section thickness and CE.
* (I) added (extra parentheses) to keep (the terms) clear in (this post); no (scathing rebukes) from (math teachers) please!