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Ok, so we’re back again to talk about steel. If you’ve seen any of the previous videos and like them, consider hitting that little mmi button and subscribing. If you’re new and this is your first video, thank you for tuning in. Or, if you’re back for more, thank you too. Super grateful you are watching. Please like the video and if you have any questions comment below, and again, don’t forget to subscribe, right here.
This video is our second on PH stainless steels. If you have not seen the first do so now. You can find it here.
So, you either have seen the first video covering the basics or you decided you know all that stuff already, so we can move on. Regardless, if you are watching now, you have the basics of the martensitic PH types covered.
Our next type is called semi- austenitic. Chemical composition of these alloys lead to a different set of structures during the steps of heat treatment.
Like all PH stainless, the first step is a “solution treatment”. That achieves the uniform distribution of the elements involved in the hardening reaction in an austenitic structure. On cooling from solution temperature, the structure of these alloys REMAINS austenitic at room temperatures……….but only temporarily. This relatively soft and ductile austenitic structure gives us the opportunity for much more extensive fabrication prior to hardening than martensitic types permit.
Well.. it looks like we found a way to have our cake and eat it too. We can have a softer and more formable metal at this stage and then we can later harden to the high strengths of the fully martensitic PH grades. To achieve the final hardening of the material we need to first make the austenitic structure able to transform to a martensitic structure. Three methods can be used to permit the martensite structure to form.
Either:
Subzero cooling to temperatures on the order of minus 100 degrees F and holding for up to 8 hrs.
or
Heating to about 1400 degrees F and for holding up to 3 hours
or
cold work ( like cold rolling sheet )
Now that we have a martensitic structure,our familiar aging treatments are able to do the final hardening In these types. The combined two step process of hardening of these grades is indicated by a letter prefix plus the H followed by the ageing temperature . The prefix indicates the method of forming martensite.
For example:
RH950 indicates a sub zero refrigeration ( hence the R) followed by ageing (the H) at 950 degrees F
TH1050 would indicate the Thermal method followed by ageing at 1050 degrees F.
CH900 indicates Cold work followed by ageing at 900 degrees F.
Common examples of semi- austenitic grades are 17-7( AISI 631), 15-7 (632), AM-350 (633),and AM-355 (634).
Applications often require the premium cleanliness of remelted steel and the precise details of the required heat treatments vary by grade and specification
Our last type of PH stainless steels are those that retain an austenitic structure from solution treatment through aging. Although much lower strength than the other two PH types, they are non-magnetic and higher strength than the 300 series stainless steels.
Solution treatment is typically at higher temperatures than the other types. Aging is also at higher temperatures of 1300 and above. Most often only a single aging treatment applies to the alloy. With the higher aging temperature, these alloys find use at temperatures where the other PH types would lose strength.
An example of this type is grade A286 with premium vacuum remelted cleanliness for aerospace engine or turbine applications.
As always we close with a caution about the critical importance of specifications in these complex materials.
For instance, I just mentioned A286 and regarding the caution surround specs, this alloy is a perfect example. There are literally 5 different types BASED ON SPECS of A286 round bar. There is an Age Hardened solution treated to 1650° to an AMS 5737. There’s an Age Hardened solution treated to 1850° per AMS 5732 and GR 660 CL B (which supersedes AMS 5735), There’s a regular solution treated to an AMS 5731 (which supersedes AMS 5736), Then there’s 2 cold reduced types one at 160 KSI to many different AMS specs, some I already listed above ones I didn’t are AMS 5734, AMS 5736, AMS 5853 and then there’s the cold reduced at 210 KSI which meets AMS 5726 as well as other previously mentioned AMS specs. SO, as you can see, specs will literally make or break your application. Never underestimate how utterly important they are and ALWAYS make sure you know what you are getting. CHECK THE SPEC!
Next up in our series on stainless steels will be on two higher strength types Duplex and Manganese substituted Austenitic types.
So, I hope you enjoyed this video and stay tuned, next Thursday, same time, same place and remember click here and please subscribe, if you made it this far, if not, please like the video. If you want to catch up on previous videos click here. Thanks again for tuning in, this is Michael with Michlin Metals, OUT!
