Story of Steel Part 6. What is Martensite?

Earlier, in Part 5 of our story, we began to take a “micro” view of steel. To this end we looked at the microstructure of a relatively “slow cooled” low carbon steel weld, defining its microstructure as a mixture of ferrite and pearlite at room temperature.

If we now take a similar steel to this, (i.e., 0.15% carbon), and do two things as follows. 

1. Increase the thickness from a “slow cooled” 12 mm plate to a 50 mm in the same steel, typically a 350WT grade.

2. Change the alloy to one of increased strength, say to a 480AT grade, by changing the alloy content of the steel and correspondingly increase the thickness to 50mm.

If we do either of the above, what may happen to the microstructure?

Case 1: Increasing the Thickness 

In case 1, since the steel is much thicker, at 50 mm, it will cool significantly faster during welding than the same steel of 12mm thickness. This faster cooling will upset the ferrite perlite mix that occurs and which we described in Part 5 of our story. 

We can recall that during cooling the high temperature austenite phase begins to change to ferrite and pearlite as the steel cools through a specific range (875C -723C). This is illustrated in Figure 1.

Figure 1. Phase diagram adding Carbon to the Iron up to 0.8%

The ferrite and pearlite only come out in the right portions if the steel is cooled relatively slowly toward 723 deg C. So, what may occur in the microstructure if we cool at a much faster rate with the 50 mm thick steel? 

What happens is that the time required for the cooling weld to change from austenite to an all ferrite/pearlite mix is not available. As the rapidly cooling steel closes in on 723 deg C, any unchanged austenite has to finally transform into something and it becomes a mass of atoms not knowing where to go to find equilibrium. Their destination has been interrupted by the speed of cooling. 

The result is we get another microstructural phase which we define as martensite and which can be very hard.So, looking down the microscope, what was once a nice simple microstructure of ferrite and pearlite is now a mixture, some of it forming a mass of needle like martensite which, even to the naked eye, looks highly stressed and, therefore, hard. 

Figure 2 illustrates the difference in microstructures from slowly cooled to a fast cooled steel. Remember that these are the same steel, just with different cooling histories. 

Figure 2. Left a Microstructure with a Ferrite/Pearlite Mix and on the Right, a Microstructure of Martensite.

Of course, it’s not all as simple as this and the final percentage of martensite formed will depend on the steel chemistry and the cooling history. However, the bottom line is that this hard martensite is prone to cracking, especially in the presence of hydrogen where HICC (Hydrogen Induced Cold Cracking) becomes a distinct possibility. 

This is the major reason why we institute preheats and why we use low hydrogen consumables and/or processes when the steel gets thicker. The intent is to reduce the quenching effect by reducing the rate of cooling and thus reduce or eliminate the amount of martensite that may form. Since hydrogen targets crack sensitive microstructures then we also need to control the hydrogen.

Table 1 illustrates the minimum preheats described in CSA W 59 for the different thicknesses of 350WT, a 350 Mpa yield strength steel, when using low hydrogen techniques.

** Welding shall not be done when the ambient temperature is below -18 deg C (0 deg F)

Table 1. Minimum Preheats for Different Thickness’s of G. 40.21 Grade 350 WT Steel

Case 2: Increasing the Steels Strength

In case 2, using a higher strength steel, the hardenability of the steel will be increased as it contains a differing mix of alloying elements to increase its strength. In this case we will move from a 350WT steel to a 480AT whose yield strength is 480 Mpa. 

By increasing the strength, we will also increase the hardenability of the steel which will result in a greater probability of producing martensite in a fast-cooling weld.

Using this higher strength alloyed steel results in the preheats as shown below in Table 2. Preheat at 12mm is essentially room temperature, but at 50 mm, and with low hydrogen practices, the minimum preheat is now 110 degrees C.

Table 2. Minimum Preheats for Different Thickness’s of G. 40.21 Grade 450 WT Steel

In a nutshell, increasing the steels thickness to 50mm in a lower strength steel results in a minimum preheat of 65 Deg C but, increasing the strength results in a preheat of 110 Deg C for the same 50mm.

The importance of preheat cannot be overemphasised, if preheat is necessary then it must be applied, and applied correctly. It is there for a reason. 

If the preheat in the welding procedure, is called out at 110 deg C then that’s what is required, not 100 deg C. Not applying the correct preheat is dangerous and could lead to HICC cracking and eventual failure if cracks are not detected by inspection. 

It is everyone’s responsibility to ensure correct practice, to ensure correct preheats are applied and to use properly conditioned low hydrogen electrodes/process’s when called for in the welding procedure. In this way we all strive to prevent failure, to keep the public safe and the environment undamaged...........we must all do our part .