What’s up guys?!?!? Michael with Michael Talks Metal back for more martensitic metal madness today. For the next two weeks our videos will introduce you to the martensitic stainless steels. Unlike the austenitic and ferritic types, a martensitic stainless can be hardened by conventional heat treatments……. you may be familiar with heat treating carbon and alloy steels. In very simplified terms, you heat the material up to a temperature where the structure becomes austenitic and then cool at a rate fast enough and “presto”… martensite is the resulting structure. Cool more slowly and the steel structure would become ferrite and iron carbides. With the faster cooling, the carbon atoms do not have time to form the carbides and become “stuck”.
Martensite is considerably harder and stronger, but can be more brittle than the other structures we have touched on in previous videos …… the austenite and ferrite types. The hardness that can be achieved goes up with increasing carbon content of the alloy. Here in part one we will be discussing alloys with a carbon content of .15 % and less which yield a hardness of 45 Rockwell C and below depending on the amount of carbon.
At their maximum hardness, these alloys are also most resistant to general corrosion but tend to be brittle. To get a combination of corrosion performance and useful strength and hardness, the material is “tempered” to make it less brittle.
Wow… so now we are balancing the hardness, strength and corrosion performance we need. These Martensitic stainless types also lose toughness in impact at low temperatures so we need to factor that in as well. Sounds complicated and best left to the engineers and designers.
Anyway, let’s get into some of the more common alloys in this category
Type 410 at 11.5 to 13.5 Chromium has a carbon maximum of .15%. Depending on the actual carbon content, the maximum hardness it can achieve varies from the low 30’s to low 40’s Rc. Various specifications restrict the chemistry or require a range of hardness in standard tests to assure the higher hardness version when needed. AMS 5612 for aircraft applications is one that requires the higher hardness in a standard test.
Type 403 is a very similar chemistry and is known as “turbine quality”… originally used for steam turbine blades, valves, and other component applications where strength and corrosion resistance is needed at operating temperatures above 800 F
Type 416 has the same .15% maximum carbon and Chromium at 12 to 14% but has Sulfur added to improve machinability. Often it has lower actual carbon content and typically is used at lower hardness than 410. Sulfur has negative effects on toughness and corrosion resistance. This grade is used where the added machinability offsets a loss in corrosion performance and mechanical properties. 416 is used in a wide variety of applications such as screw machine parts.
As we have said before, specifications often restrict the chemical composition further and add requirements for the material far beyond just the chemistry. Specifications and part requirements can alter what we have shown today so … for the 141 st time. CHECK THE SPECS!!!!! TWICE
Stay tuned for part 2….for the higher carbon – higher hardness martensitic stainless steel alloys next week. Thank you for watching. This is Micahel with Michael Talks Metal, if you need any more info on these martensitic grades, check the website for more info www.michlinmetals.com
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