I would shed my contribute in spirit of cooperation, that Gianni has already experienced, basing on Professor Emeritus Verhoeven and Dr. Landes publications, not to mention many other in depth publications that I already sent to Gianni on 8/8/2014 via email.
All of this material has been sent years ago to Elliot Williamson and Neels Roos as well .
The former used the HT data per my specs to realize not only three folders + 1fixed blade in M390 but also a kitchen knife in Elmax for me, but the guidelines had been used from then on also for other highly alloyed steels, notably S90V.
Neels Roos defined all of this literature ("Now, what a read..."), which for me it is a pride and honour.
These literature involved Elmax and M390.
I had request on Britishblades of disclosing the HT privately to some EU customakers, which I did gladly. Mark Hill of UK, and some others from Poland.
With this being said, and limiting to M390 (as the story for Elmax would be pretty much the same) I would signal to you my posts in this thread
http://www.bladeforums.com/forums/s...n-Edge-Retention-cutting-5-8-quot-rope/page47
Here I made some strictly metallurgical remarks that I don't feel like duplicating here (exception made for SH) and notably I put the link to BU m390 datasheet from which you can get clearly the message that M390 delivers its best when oil quenched.
But I acknowledge that this NOT a viable solution for production blades, so we should limit to vacuum HT.
PART 1
A publication from BU
"CARBIDE DISSOLUTION RATE AND CARBIDE CONTENTS IN USUAL HIGH ALLOYED TOOL STEELS AT AUSTENITIZING TEMPERATURES BETWEEN 900 ◦CAND 1250 ◦C" S. Wilmes and G. Kientopf, Uddeholm GmbH, Düsseldorf, Deutchland is quite interesting to get an idea, among other steels, of M390 behaviour.
In as
annealed state it has the follwing carbide composition:
Amount of Undissolved Carbides =>28.6%; 91 % Cr23C6; 9 % V4C3 (percentage expressed in weight);
In the as
quenched at 1200°C we have 17,6% undissolved carbides Cr7C3; V4C3 (small amount)(percentage expressed in weight);
So we've already two results.
1)39.5% less undissolved carbides, which will help us having more available C and Cr in solid solution for hardness and stain resistance. Make a bookmark to the latter, as later it will help us (Secondary Hardening).
2)Weaker Cr23C6 carbides are gone (that's why we have more C and Cr in solid solution) and we've only K2 Cr carbides (the hardest of Cr carbides) and MC V carbides (even harder).
Not only Cr23C6 carbides are the weakest of Cr ones, not only they bind a hell of C and Cr, but they are also responsible of Intergranular Corrosion.
Lets explain what it is (
Verhoeven, Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel pages 137-139).
"When a small particle of K1 (Cr23C6) forms in a matrix of either ferrite or austenite, the Cr atoms in the carbide are subtracted from the matrix iron immediately surrounding the particle. If this causes the %Cr to drop much below 12 %Cr in the surrounding matrix the steels become susceptible to corrosion and are said to be "sensitized". Since the carbides prefer to form along the grain boundaries of the matrix the corrosion occurs along the grain boundaries and this type of corrosion is therefore called intergranular corrosion. Hence, sensitization is said to occur when a low temperature heat treatment, such as tempering, causes K1 precipitate particle formation to the extent that intergranular corrosion may occur"
He then mentions 440C to make a practical example pinpointing the fact that at 1100°C and upon quenching we will have Martensite + K2 (Cr7C3) carbides.
What 440C achieves theoretically at 1100°C M390 achieves at 1200°C (15' for a knife blade). To be picky it would be 1180°C when NOT using a vacuum furnace.
PART 2
QUENCHING
When Vacuum HT is used, Positive Gas Pressure must be used also as far as quenching is concerned: BU says
minimum 3.5Bar. I would hotly recommend 5Bar.
CCT graph available in Elmax own datasheet could be used to have a very good quenching timing for M390 as well, taking into consideration the higher Aust temperature.
I.e.: roughly 300°C per minute (4mm specimen) until 550°C then room temperature cooling until 70°C. Followed immediately by tempers.
Now, it is true that M390 is austenitized at 1200°C (against 1100°C of Elmax) but it has 0.6% of Tungsten which:
1)Decrease the critical quenching rate to avoid Bainite nose, or if you prefer shifts the bainite nose to the right of CCT graph.
2)Gifts its contribute to "Particle Drag" (Prof. Verhoeven, ibidem), thus limiting furtherly grain growth together with V and Moly in this steel.
So 230°C per minute until 550°C and then room temperature until 70°C.
PART 3
TO CRYO OR NOT TO CRYO?
My quick answer is negative.
Lets elaborate: M390 has the advantage of having a very very rewarding secondary hardness range. Cryo will solve only the ratained austenite problem.
SH will solve RA and possible Fe3C (Cementite, very hard but brittle) problem all in a row.
PART 4
TEMPERING
I've anticipated that SH on M390 solves two problems. RA and residuals of Fe3C (AKA Cementite). This is possibly why also S90V/comparable grade PM steels perform very well after SH: i.e. greatly reduced microchipping, as Elliot Williamson has remarked.
It is true that some Cr will be tied up in secondary Cr carbides, but V and W which are stronger carbide formers will limit this.
"Secondary hardening is caused by the formation of clusters of atoms of alloying elements and carbon (a maximum hardness often corresponds to the clusters) and the replacement of
relatively coarse particles of cementite by much more disperse precipitates of special carbides (TiC, VC, Mo2C, W2C). When these particles coagulate, hardness decreases.
The chromium additive causes a small secondary hardening. This is connected with a rapid coagulation of the Cr7C3 carbide at 550C (10208F) as opposed to Mo2C and especially W2C. During secondary hardening an increase in the yield stress is accompanied by an increase in toughness owing to dissolution of coarse cementite particles..........Secondary hardening is a result of the transformation of RA to martensite on cooling from the tempering temperature, and of precipitation of an ultrafine dispersion of alloy carbides.
Tungsten, vanadium , chromium, and molybdenum that are the strong carbide-forming elements are most commonly used to achieve secondary hardening.
To take advantage of their precipitation characteristics, they must be dissolved in austenite during the austenitizing treatment in order to be incorporated into the martensite formed during quenching with sufficient supersaturation for secondary hardening during tempering."
Source: Engineering - Steel Heat Treatment Handbook - Metallurgy And Technologies, 2Nd Edition - (George E Totten) Taylor & Francis Crc Press 2007
I know two possible remarks:
1)Decreased stain resistance.
2)Decreased toughness.
I will not hide. Both are true.
From my sources I have evidence of differences in corrosion resistance in Solution A 5% HNO3 -1% HCl / 3 hours / room temperature
Solution B 10%CH3COOH / 24 hours / boiling environments. Let me know, please, if you cut in such environments for such long times.
Normal stain resistance it is quite less demanding and I've never had an M390 blade (all SHd) staining either during my Alps or my Tuscany trekkings.
Toughness wise, when M390 is Austen. at 1150°C ant SH at 540°C it has ca 35J of toughness against 43J it would have had if tempered at 250°C.
Source:
PMPLASTICMOULDSTEELSWEAR RESISTANT AND CORROSION RESISTANT MARTENSITIC CHROMIUM STEELS
C. Kerschenbauer,M.O. Speidel,Institute of Metallurgy,ETH Zuerich,Switzerland,G. Lichtenegger, J. Sammer and K. Sammt,Boehler Edelstahl Kapfenberg, Austria
These are the conditions needed to HT M390, would you wish to ACTUALLY see what it delivers