The full transcript for the video can be found below. In this video we are trying to answer the following questions. What is martensitic stainless steel? What is the difference between martensitic and austenitic stainless steels? What grades or types are considered martensitic stainless steel? Can martensitic stainless steel be heat treated?
In our previous video on martensitic stainless steels part 1, we talked about types that have a maximum carbon content of point 15% and hardness in the low 40’s and below. If you did not check out that video please do so now.
Ok. So, if you didn’t just watch that video here’s a quick review and some takeaways. Higher carbon content allows us to achieve higher hardness and strength with heat treatment. Higher Chromium and other elements can improve corrosion performance and / or mechanical properties.
This discussion covers the types where hardness would be from the mid 40’s to the 60’s on the Rockwell C scale before tempering.
First up are several grades that are small modifications of the chemistry we saw in type 410 which we discussed in the prior video. They have similar applications but are utilized where 410 is not quite satisfactory:
Type 414 at point 15% max carbon and 11.5-13.5% Chromium has Nickel added at 1.25-2.50 % which improves its ability to harden and adds toughness that makes it more useful in the low 40’s Rc hardness range than type 410 which we discussed previously. The Nickel also improves corrosion performance in some environments. In addition, it improves mechanical properties at modestly higher temperatures as compared to type 410.
Type 418 (aka Greek Ascoloy) at point 15 to point .20% carbon, 12 to 14 % Chromium, 1.8 to 2.20 % Nickel, and 2.50 to 3.50 % tungsten for higher strength at elevated temperature above those where 403 and 410 are used. Aerospace applications in jet engines or land-based gas turbines would be examples.
Type 422 at point 20 to point 25 % carbon, 11 to 13 % Chromium, point 5 to 1 % Nickel, point 75 to 1.25 % Molybdenum, point 15 to .30% Vanadium and point 75 to 1.25% Tungsten. Say THAT ten times fast! So after all that, 422 is used for the complex requirements at elevated temperatures.
Next up is a type that could easily be considered as its own group of alloys
Type 420 is very broadly defined as point 15 MINIMUM carbon with 12 to 14% Chromium. Now, this is the first time we have come across a grade with only a minimum carbon specified in these videos. We know how important the carbon content is in martensitic stainless steels so we can expect a wide variation in hardness and strength depending on the carbon content. Hardness can range from the low 30’s to middle 50’s on the Rc scale.
In the most common compositions, the carbon is in the range of point 3 to point 4% percent. Common applications are in cutlery and plastic molds. There are also free machining grades indicated by the “F” suffix… 420F. ALWAYS be looking for a further specification that defines the material to assure the final product will do what it needs to do in service.
Lastly, we have a group of alloys in the highest range of carbon in the martensitic series, the 440’s
Chromium content is 16 to 18% and has three variations with differing carbon content. The higher Chromium content improves the corrosion performance over type 410 or 420 and the higher carbon increases the strength and hardness.
440A has .point 60 to point 75% carbon.
440B has point 75 to point 95% carbon.
440C has point 95 to 1.25% carbon
Hardness from the low 50’s for “A” and mid 50’s for “B” and they are commonly used in cutlery. 440C is typically used at around 60 Rc and is utilized in valves or bearings and other applications where both corrosion and wear resistance are important.
As usual, we caution viewers that chemical composition can be further restricted by specifications and add important additional requirements. Parts that need a prolonged resistance to fatigue will often specify a premium version that has been melted and refined a second time under a vacuum to remove any residual impurities that may remain in a single melted alloy. More on that in future videos.
We appreciate you tuning in, please don’t forget to like the video. If you have yet to subscribe, please do so here and don’t forget, if you need a review of the last videos, click here. Have a great day and thanks for watching. This is Michael Michlin Metals, OUT!