What’s up guys? This is Michael with Michael Talks Metal back for another talk on some metal by golly! I mean what else? Do me a favor, click this little button and subscribe it will help the channel. Thank you and I appreciate it! Let’s jump in!
Over the last several weeks we have been talking about special types of alloy steels. Today’s video will explain what it means when an alloy like 8620, 4140, or 4340 has an “H” suffix added and becomes 8620H, 4140H, and 4340H.
First we need just a bit of background stuff….
Alloy steels like these develop their best properties after heat treatment. How do we compare alloys and know what to expect after heat treatment?
Alloys are very different from each other. Even variations within the allowable chemistry limits of an alloy can affect the hardness profile that can be achieved.
Metallurgists had to develop means of evaluating the hardenability to be able to compare the response of a given alloy chemistry to heat treatment.
One method, the “end quench” or “Jominy end quench” hardenability test… originally used a sample of the material ….. about one inch in diameter and four inches long. It was hardened by heating to a standard temperature and then quenched from one end of that cylinder. The sample then was hardness tested at increments of one sixteenth of an inch along the length of the cylinder starting at the end that was quenched.
Results are expressed as a Rockwell C scale hardness at the distance from the quenched end. For example “J1 = 40″ is a 40 Rc hardness one sixteenth from the quenched end and J8=30 is 30 Rc at eight sixteenths from the quenched end.
Today, the hardness profile can be accurately determined by computer from the very precise chemical composition of modern steel making procedures.
H steels establish standards for the hardness profile of the steel that then assures a more consistent and repeatable response to a heat treating process from batch to batch of an alloy. Depending on the alloy, each sixteenth inch position may have a minimum hardness or maximum hardness or both a minimum and maximum requirement.
When even more restrictive requirements are required there are also standards for Restricted Hardenability alloys… with an ” RH’ suffix added to the standard steel designation.
Imagine an automotive company producing many thousands of the same part. They want the same results from batch to batch without changing their heat treating process.
Chemical composition is permitted more variation in a standard “H” steel as long as the hardenability band limits are met. H band steels do not necessarily meet the chemistry requirements of the regular grade and the regular grade end quench profile may not meet H band requirements.
ASTM A304 is the standard for H and RH alloy steels
ASTM A255 is the end quench procedure standard .
End quench hardenability restrictions often appear in other specifications as well and we always caution you to
CHECK THE SPECS…
That’s it for today folks. Thanks for tuning in, especially if you made it this far….and if you have, please consider subscribing. If you missed any previous videos, click here. Thanks again for watching and this is Michael with Michael Talks Metal. OUT!