Why would you normalize, austenitize, quench, and temper 4130/4140 9

[valvesarefun]

Hi all,

I have a customer who has specified the above heat treatment procedure for some forged material. I am interested in getting some opinions on what purpose the normalizing might serve. We typically only preform the latter three heat treatments and end up with a product with good properties. These are pressure retaining piping components for general oilfield use. So my basic question is what difference is the normalizing going to make and does the difference justify the extra furnace time?

Best regards,

VRF

 

[redpicker]

The temperatures used for forging will generally result in a very large grain size. This, and uncontrolled cooling rates from forging temperatures can result in a microstructure that does not respond well to heat treatment. Performing a normalizing operation before the austenitize, Q&T will often result in deeper hardening and improved toughness properties.

Yes, you can often meet the specified mechanical properties without the normalize operation, but in all likelihood, your customer is not just interested in meeting the specified properties, but wants assurance that the highest strength and toughness combination possible are achieved, and you get that with including a normalize operation before the hardening operation.

Is the improvement worth the extra time? Obviously, your customer thinks so, which is why he specifies it. Since he is the one paying the bills, I'd have to say the answer to this question is "Yes".

rp

 

[metengr]

Yes, as a customer we specify it for the reasons above.

 

[valvesarefun]

Thank you for your answer redpicker that makes sense. Certainly since they are paying for it I'm willing to do it, but the reason I'm asking is to find out if the difference is profound enough to cause me to change our own procedure. I guess I'll just do some testing to see if the end result justifies the added expense.

For my own knowledge I am curious as to why normalizing increases a part's response to hardening. Is it all about refining the grain size? "the book" says that austenite nucleates and grows from pearlite, so if normalizing achieves a mostly fine pearlite structure, with some lesser portion being bainite, is that why the part will respond better to the austenitization+Q&T ? I also notice that the literature and you were careful not to say that the "hardenability" is increased. Would you expect a different Jominy result from a previously normalized part, compared to one that was not normalized?

As always thanks for your time.

Best Regards,

VRF

 

[redpicker]

No, it's not "all" about grain size, but that is part of it.

As mentioned above, forging temperatures (>2300 F) are high enough to cause a large grain structure. Also, the cooling rate from forging temperatures is often not controlled. Often, the parts are just thrown into a large bin off the forge until the bin in full. Once the bin is full, it is often just left alone until the next day and they are fully cooled. So, all of the parts have cooled slowly, but some of them could have very well spent an extended amount of time at a temperature where both austenite and ferrite are stable. Because of the large grain size, this can result in considerable amount of micro-segregation of carbon, with large ferrite grains and an austenite decomposition product that can be anything from a coarse pearlite to spheroidized carbides. What makes matters worse is that nearly all of the carbon, and, depending on the alloy content, a lot of the alloying elements, can be tied-up in these carbides.

This structure can be very difficult to austenitize since the ferrite is very low in carbon, so it has to either get above 1700F or so or absorb carbon from neighboring carbides or austenite (that has already absorbed carbon from decomposed carbides). As the ferrite grains can be large, and the carbides can be large and spheriodal, this can take a surprisingly long amount of time.

One problem this causes is that not all of the forgings in the bin will have this condition. Some of these parts will cool rapidly enough so when they will austenitize easily and the only effect will be perhaps a slightly larger grain size than would have been obtained if a normalizing operation had been performed. Others, however, may have this problematic structure. The result is that if you heat treat a batch of 100 and select one for destructive testing, it may produce good results, but others in the batch may not have responded as well.

A normalizing operation will make sure all the parts have the same starting microstructure, eliminating the problem.

If you control the process, you may be able to eliminate this problem without needing a normailzing treatment. At my first professional job, we had a set-up where the parts came off the press onto a controlled cooling conveyer and were not put into the bin until the temperature was below 800F. After rough machining, the parts were given a Q&T and had to meet very demanding tensile and impact properties. The process was tightly controlled and produced very consistent results. We were producing our own parts, however, and we only had to satisfy our own engineers, not a host of engineers from various customers.

rp

 

[valvesarefun]

Redpicker, thank you for the thorough explanation. I appreciate it greatly!

Best regards,

VRF

 

[salmon2]

Excellent discussion by redpicker. I as a customer often specified this but not fully understand this.

 

[gearcutter]

redpicker - thanks for taking the time out to explain that to us.
Star for you.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[rmw]

I'm in for a star for Red too. Thanks for taking the time to spell all that out in very understandable detail.

rmw