|Sam Stones||29/09/2021 01:24:25|
835 forum posts
As a follow up to Andrew and Tony’s comments, I couldn’t resist telling the following story.
I was into my 2nd or 3rd year of my apprenticeship when a rather precarious situation developed.
The toolroom heat-treatment plant (a closed off section of the workshop measuring about 8 metres by 6 metres), was equipped with various gas and electric fired cyanide furnaces. A basic gas torch and a brick hearth, along with a bucket of whale oil were there ‘for personal use’.
Quenching of the major throughput was either into a warm water bath for smaller parts, or a very full tank of oil. This tank measured about 150cm deep and was roughly the same diameter. One of the older toolmakers was responsible for all of the once-a-week ‘serious’ heat treatment.
Almost full, the large oil bath became a considerable (flash) point of interest when a large and very hot piece of tool steel (an insert for a compression mould) was being quenched. For scale, the piece (effectively one of several thick-walled cylinders) measured about 150mm diameter and some 400mm long. It was bored out about 45mm diameter through its entire length.
Transferred with some difficulty from a cyanide bath to the hand-operated hoist, the hoist failed (jammed) while the hot insert was half in and half out of the oil. It doesn’t take much imagination to visualise what happened next.
During the initial struggle to free the jammed hoist, the oil began to boil and instantly caught fire. Flames quickly crept across the surface and out to the edge of the tank. Within seconds, flaming oil was dripping onto the floor.
Of greater concern was what was on the other side of the brick wall and thin cement sheeting. It was the powder room known as the drugstore. It supplied measured amounts of rubber, sulphur, and powdered coal to several Banbury mixers and two-roll mills.
Having, for a short while peered through the open door as a not particular welcome spectator, it was time to make my exit. The foreman and a couple of others succeeded in releasing the jammed hoist, quenching the steel, and extinguishing the fire with sand etc. The fire brigade arrived to find the toolroom filled with smoke.
I suspect the other cylinders were subcontracted.
Perhaps that's why I prefer a water quench
|Ramon Wilson||29/09/2021 08:03:13|
1194 forum posts
Perhaps I should have added but like so many other matters in life 'size' needs taking into consideration.
A well damp cloth or asbestos kitchen blanket laid over the quench bath will quickly douse any flash fire but as said depends on the tank size. At home mine is quite small - 6in or so diameter but more than enough for what I do. At work it was about 18" diameter and slightly deeper. With the amount we did at work (mainly batches of small components but occasionally a larger lump) it would often flash over but was quickly doused as above - I honestly can't recall that happening at home even before I had whale oil - then it was old car oil which will readily flash if got hot enough.
Personally I prefer to use oil for the result - not to eliminate the fire risk but each to his own eh
|Robin Graham||03/10/2021 23:56:20|
|874 forum posts|
Thanks for replies and apologies for not getting back earlier.
From the replies it looks like I can carry on brine quenching without too much fear - the the thickest section I'm using is 3mm by maybe 40mm width. I had worried that I was on the threshold of distortion/cracking disaster, but it seems not.
I have Tubal Cain's excellent book , but in his treatment of quenching he seems (for the most part) to talk about high carbon steels in general. However, in the last paragraph of p32 of my edition he says:
"For model engineers, therefore, we can say that in general water will serve for our needs, but brine is preferable when ultimate hardness is needed. Oil should be used for milling cutters and for gauges or hardened jigs - preferably using the proper oil-hardening steel for the latter as it is formulated to avoid distortion [my bold]."
My assumption was that this steel (eg O1 I guess) was optimised for oil quenching and wouldn't perform as well as 'normal' tool steel (eg silver steel) when water quenched. Otherwise, why not make all tool steel to this formula? I'm still not clear about that!
No-one has commented about Sparey's suggestion of 50% suds as a substitute for whale oil; If I get time (unlikely!) I'll give it a go and report back.
Edited By Robin Graham on 03/10/2021 23:57:19
|Ramon Wilson||04/10/2021 08:27:57|
1194 forum posts
Personally, despite being an advocate of Sparey book I wouldn't use soluble oil/water mix myself but, like so many things, unless you try you don't find out. Trying to get B01 harder than 60 - 62 HRC at home by overheating and or very cold quenching will, in all probability, lead to crystalisation that tempering may not improve.
Actually Robin I did but as said, though I personally would not try it doesn't mean you shouldn't.
The thinner the section the easier it is to distort on quenching. Any thin part needs to go into the quench absolutely vertical and plunged firmly but quickly. Just dropping the part in will cause distortion as one side cools before the other. The other thing to bear in mind with thin sections is the ease in overheating them in the first instance and the part's inability to heat unevenly if using a flame.
Can I ask what it is you are making and what it's intended use is? BO1 is intended as an oil hardening steel (see Uddeholm spec for it here but can be treated in other ways. You haven't mentioned tempering - making parts to high hardness (and if done by flame always a potential) - tempering will not improve any crystalization caused by overheating. I should add though, one has to really overheat it to get to that state.
Much has been written in modelling circles on the technical side of heat treatment over the years - all of it valid but I offer my thoughts based on not only on practical experience at work but successful heat treatment at home particularly on making cutting tools.
As an aside I made some small LH thread retaining screws from silver steel for a series of IC engines. These were carefully heat treated (quenched in oil) and tempered. It would be a while later that I had cause to strip one of these engines. The screw removed easily enough but when handling to replace simply broke in my fingers at the thread shoulder, the material completely crystalized and crumbling. Obviously the other engines were checked and all other screws were okay but they are all suspect and require replacing.
Edited By Ramon Wilson on 04/10/2021 08:29:19
1207 forum posts
All the tales of heat treatment reminded me of a story once told to me about the heat treatment area in an engineering concern making reasonably small items. A new manager had been appointed and he toured his new domain to inspect what and how was being produced. When he came to the heat treat area he found it was the domain of a long-standing employee and he was not impressed by the cleanliness or lack of in this area, he was particularly unimpressed by the state of the quenching tank which contained water that had not been changed or refreshed for a considerable time, he ordered that it be drained and refilled with fresh water. Subsequent to this water change the product was not achieving the hardness levels that had been the norm prior to the water change, when questioned as to why this was so the long-standing resident of this work area imparted the information that every morning before starting work he would relieve himself in the quenching tank, one can only assume that the build up of salts from his daily urination had a significant effect on the way in which the quenching medium affected the achieved hardness. Dave W
|Howard Lewis||04/10/2021 14:48:39|
|5298 forum posts|
My recollection of heat treatment areas is that the foreman carried a little book in his pocket, as a reference manual, to his many years of experience., Something along the lines of "That should have 30 minutes, but best to give it 35"
Or the need to quench really suddenly, and in a particular orientation, as Tug says, to prevent distortion.
Sometimes more black art than precision, to get the job right!
|Mike Poole||04/10/2021 15:38:47|
3071 forum posts
At tech college they had a drum of oil for hardening, a friend discovered that a red hot bar about an inch diameter would vaporise a nice cloud of oil vapour before it had lost enough heat to avoid ignition of the cloud when withdrawn from the oil, this was highly entertaining until the instructor appeared, trying not smile he shook his head in disbelief, the entertainment was over with no doubt in our minds that this was not an approved activity.
|Sam Stones||04/10/2021 22:07:12|
835 forum posts
Thanks for an interesting thread Robin. Apologies for any repetition.
It’s too long ago and I no longer have the endurance to try to reiterate what I learned in ONC and HNC Mech Eng, but for a theoretical insight into what is taking place, take a peek at this -
Someone here might like to highlight in simple terms, the basics of the rate-of-change from FCC to BCC and the way it influences properties.
At a practical level, I’d recommend attention be given to –
Incidentally, it may be a surprise to discover that Young’s Modulus of Elasticity hardly changes.
Have fun, especially with your tests, Robin.
|Ramon Wilson||04/10/2021 23:18:17|
1194 forum posts
Well Sam I don't have the honour of being able to say I have an ONC or HNC Mech E to my name (congratulations BTW) but I have placed an awful lot of small pieces of tool steel of differing varieties into a quench in order to heat treat them to specified and very defined limits. Despite my lack of academic prowess, for some fourteen years I had the pleasure of running the machine shop for a company that produced electric terminals on multistage progression press tools.
In my experience of such it is not essential - to the average home ME - to know what the technical aspects are behind the process but knowing what and what not to do on a practical level is. To me this is far more important to be aware of in obtaining an acceptable result in those conditions. Each to his own though - I guess I'm far more interested in knowing 'how' it works than 'why' but I stress - in the home workshop environment.
The heat treatment of parts I was responsible for making were mainly punches and anvils/dies in a variety of tool steels for small press tools and parts for the progression press tools. Many of these were quite thin and liable to distort if not handled carefully but they were hung from MS cages - about 10/12 per cage and four cages at a time in the two ovens to be brought to temp. Each cage would be quenched by hand. Sometimes this could be on a daily basis but usually on a weekly one - apart from the silver steel parts previously mentioned I don't recall we ever had a failure
The punches were usually BO1 and the dies from a different steel. All were then ground to fine limits on thickness and width and the punches then form ground in blocks to create the punch/die profile.
I have never felt that the technical aspect has ever been a help in producing a well heat treated part in the home workshop - but that's just my approach, others of course may see it through different eyes
Best - Tug
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