[TheForge] Re: Welding 5160 (Ralph Sproul)

[jim wallbridge]

-- 
Tom Troszak, said in part::
"In actual practice, blocks of H-13, 3 inches thick pulled and quenched as
soon as color matched furnace temp, rarely got above R 40. Blocks soaked for
an hour after the temp spike and quenched get to R 50 or R 55 no problem, so
"something important" definitely happens during the soaking, long after the
outside of the blocks reached critical temp. Perhaps someone on the list can
explain "why", I just know "what happened".

Below one inch of thickness, however, I can find little reason (in my
experience) to soak any appreciable length of time once the critical temp is
reached. I have quenched 1 inch dia. punches as soon as they reached
critical temp, and I have let them soak for 20 minutes at critical temp, and
could not discern a difference in the final product.

Also, according to the tech guys at Uddeholm (whom I hold in high regard), a
lot of action takes place between 350-150 degrees F. during the quench, so
they advise quenching all steels to room temp. before tempering.

I am not an expert, I have just hardened hundreds and hundreds (and
hundreds) of pieces of tool steel, and I keep good notes, and this is some
of what I have found out.
Also, the recommended critical temp for a given grade of steel varies
noticeably from brand to brand, even from batch to batch. The product I got
from Crucible was vary variable, and even though it was cheap, it was too
frustrating to deal with on a production basis. The product I got from
Uddeholm was incredibly consistent, and never varied from batch to batch.
If you are buying steel from a reputable manufacturer, ask then for the heat
treatment data, and follow it to the letter. If you are using steel from
scrap, you will have to experiment to find the sweet spot. If you only have
one piece, you cross your fingers and hope for the best".


In replying I am simplifying an extremely complex subject and the for 
that reason the following may not be fully correct in all cases. 
When considering necessary time to hold at temperature for 
austenitizing: H13, S7, 5160 and "tool steel" (I am assuming  W1 or 
O1 herein) represent a very wide range of effects.  Based on work I 
did for my master's full transformation occurs in fractions of a 
second for low carbon steel and therefore it would only be necessary 
to hold until the center of the steel had reached temperature. In my 
work temperatures due to overshoot, I was using heating rates of up 
to 3,000 F/sec, were somewhat above normal for the steel. For simple 
carbon steel (W1 and perhaps O1) a somewhat longer time is required 
for the iron carbide to be dissolved which will depend on carbide 
size (i.e.a full spheroidal annealed structures with large carbide 
particles may take a few seconds.to a few minutes. For 5160, S7 and 
H13 we need to dissolve chromium carbides which is not easy 
(approximately 1,  3 and 5 % chromium respectively)  The time 
required to dissolve substantial quantities of large chromium 
carbides particles can range from minutes to many hours and is even 
impossible for large percentages of chromium at lower temperatures. 
In the case of S7 and H13  higher temperatures (approximately 1725 & 
1850 F) are necessary to get most of the carbide into solution in a 
reasonable length of time.  In the case of the HSSs and the D's it 
may not be possible to get most of the chromium carbides into 
solution even at the 2300 F+ used for some of these alloys. For 
compositions with high chromium contents it is not considered 
necessary to get all the carbides dissolved for many applications and 
that is standard practice.  Higher temperatures are cheaper than long 
times for heat treaters and are usually employed for that reason.. 
The reason that it is important to get the carbides into solution is 
that is the only way that we can get the benefit of the chromium as 
regards hardenability etc.which is a primary reason for its presence 
in all these steels and if the carbides are not dissolved there is 
little carbon to harden the remainder of the steel..I would suggest 
that chromium content be consider a primary variable when determining 
austenitizing time.  For scrap I would suggest it is important to use 
spark testing to check for chromium content in order to set heat 
treating parameters.

I would agree that a lot is happening at 300 to 150 F during cooling 
and in fact tthis importance continues down to below room 
temperatures in some cases (i.e. liquid nitrogen treatment to reduce 
retained austenite... .

Tom, I would be very interested in what alloys, in what forms, 
particularly if it is a W or O tool steel, you having been buying 
from Uddeholm and Crucible if that is not privileged information.

jim
(James ME Wallbridge P. Eng.) Libertarian, Metallurgist, Wood & Metal 
Worker, Fly Fisher,  Resident of Calgary,  Canada's new head office 
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