hardening and tempering leaf springs

I managed to find some sensibly priced material, on ebay naturally,. It was described as "annealed spring steel strip"..I made up the springs as specified by M Evans, and to see what happened, i tried hardening and tempering with a blowtorch, on a sample!. well, the results were not good, serious distortion, due, is suspect, from uneven heating, and the end result was extreme embrittlement. I could hardly handle them without they shattered. I have been following the techniques as set out in the Tubal Caine book on the subject, and following the advice within, decided to construct a temperature controlled furnace. This completed, with a fancy temp controller, tried another sample spring. I'm hardening at 770c, and tempering at 340c. Well, it was hard alright, shattered when I bent it gently, but undeterred, went ahead and tempered it. Some success, it was a spring. Duly encouraged I put one of my finished springs in the furnace, started it from cold and took it up to 770c for the recommended 40 minutes ( 1 hour per in thickness) When it went in the spring was only slightly curved, it came out a semicircle! and, having quenched it I placed it on the bench and is i watched, the top leaf broke in three bits... Undeterred, I cooled the furnace down to tempering level and gently put the spring in. 10 min. soak and into the quench. Amazing, I had a spring!! albeit a most weird shape.. How can I prevent the initial distortion? I had thought of bending a chunk of 1/4 flat bar to the required shape and clamping the spring to that before hardening. Any comments would be appreciated..Tom.

Don't buy material from Ebay if you want reliable properties. Hardening temperature seems a tad low, I'd go to 780C. How thick is the material? Soaking for 40 minutes seems a long time. For rectangular sections it's the smallest dimension that sets the soak time, not the largest. Tempering temperature seems a bit high, I'd be around 300-310C. Can't help on the distortion as I haven't experienced it. Poor material may be?

Andrew

Tom,

I made a somewhat larger leaf Spring a while back for a 6" TE. I bought the material direct from an accredited Sheffield supplier (at some expense due to minimum order charges) 1/8" thick, 1 1/4" wide. I hardend it using a combination of large propane torch and a wood fire. Put in the fire first and then brought up to heat with help from the torch. So brought up to temp fairly slowly and then quenched in brine (genuine direct from the North Sea! - well Thames Estuary at least). Te spec I got from the supplier suggested oil quench would be kinder but I would have needed a lot of oil for a spring that size! The individual leaves (I did them one at a time) did change their curvature a bit in the process but not to the point of being unusable when assembled. I can confirm after hardening and before tempering they were glass hard and I also broke a leaf into 3 bits. For tempering all I did was polish them up and then stoke up the wood stove again and put them on a rack over the fire and watched the colour - took them to a dark blue. Worked fine and had spring to it. Sadly I had miscalculated and I needed it to be slightly stiffer so I intend adding a couple of thicker leaves to beef it up a bit, not got round to that yet though as its not ready for the road!

A mate did similar and he also found the leaves changed shape so he made a jig as you suggest to restrain them during the hardening process - which worked.

As Andrew says the soak times are based on thick sections to ensure an even heat all the way through so in the sizes you are using you can pretty much disregard that, a couple of minutes should be fine.

Paul.

Thank you both for your comments. Buying from ebay was perhaps not too clever, but the cost of material from most suppliers was around £250 for the six springs, and I just couldn't justify that level of expenditure after all I have spent on this long drawn out project! ( Highlander Black five). The temperatures used were as specified by Tubal Cain in his book, and my furnace keeps the temp within 3'. I'm sufficiently encouraged to try again with a jig, the soak time by the way was based on the spring pack being 3/4 x 3/4, and since i had them firmly bolted together, I assumed that I should regard them as a solid block. As there are 180 leaves, I just couldn't face doing them all separately... Thanks again, I'll let you know what happens !! Regards, Tom

Tom,

You could try BSS "call 0114 244 0527 or drop us me email [email protected]" they are a division of Fernite Sheffield. I bought my strip through Fernite and I had to buy 18' to reach the minimum order charge. However since then through BSS they seem to have abolished that although you would have to talk to them to confirm (I have not dealt with them since they just keep emailing me!). Fernite do have a bit of a technical department if you can reach them! Got some useful info from them for my job regarding temperatures and specs.

My friend who made the restraint jig used a lump of thick plate with a curve cut on the edge so it was stable in the required direction. 180 leaves is quite a project! I do think stacking them as described may cause problems with the quench in the middle of the leaves as the outside will cool a lot quicker than the centre, edge on that may cause some internal stress and explain your distortion? Restraining like that may just lead to more cracks. Painful though it may be I think probably pairs at the most would get the best result. Be interested to hear how you get on.

Paul.

I’m not a smithy, but I think I would make a mild steel jig to constrain the ends of the leaf and go from there. Perhaps a couple at a time, of each size (dependent on leaf/jig and kiln size).

Won’t take so long - it is only the hardened quench that is important? Dunk the jig/leaf and load it up again. The tempering would not actually need a quench, if done at a controlled temperature, so that could be started and left to complete on its own with the kiln fairly full of parts as long as it is heated slowly....

As per Andrew, 770degrees Celsius seems a bit low - things can lose temperature fairly fast between kiln and quench, I would have thought? For thin sections time of soak is more important as, at red heat, the steel will lose carbon from the surfaces in an oxidising atmosphere.

I've successfully tempered oil-quenched springs, admittedly of small size and thickness - for the Ken Swan-designed 'Wren' - in molten lead , having verified the temperatures.

For quenching I heated the leaves, threaded onto wire, above the oil-tray and just dropped them in so they had no chance to cool significantly in the air.

The lead-bath was just a washed-out food tin, with about an inch of melted lead. I kept the lead just on crystallising point, raising it slightly to draw the springs out after several minutes of soaking.

Noticeably, the submerged steel stayed bright. Steel protruding from the lead turned blue/purple (oxidising), indicating the process was working.

BTW... I did this all outdoors!

Though an idea that came to me, I don't claim originality. I knew industry uses molten salts of various types. The owner of a small-scale steam-lorry telling me he'd used a lead-bath for its road springs. (Salts - common salt melts at about 800ºC, but that is too high for tempering.)

I can see two issues with that. One, there will be a variation in quenching due to mass and the inner leaves cooling via conduction to the outer leaves. Two, the recommendation for leaf springs is that all leaves have the same radius before assembly so they only touch at the ends before clamping. Clearly this won't be the case if the leaves are hardened and tempered together. So the spring may not behave as expected.

Andrew

Thomas, plus one for spurious metal supplies although not all embay suppliers are so but make sure they are a recognised UK company. You may consider M-machine metals, they do spring steel strip but the spec is not stated so ask.

Over the last 40+ years I have made a great many flat and V springs, I use either EN45 or EN47 IMO you temperature's are good at 770/780*C for hardening and 340 C for tempering. Many books say 300 C for spring temper -- they will break !

I would suggest you heat treat all the leaves separately and not as you have tried in an assembly, as Andrew says doing so will cause you problems. The soak time for hardening when done separately will be relatively short -- maybe 10 mins in a preheated muffle, quench in oil defiantly not water. temper as soon as your furnace drops to the required level and I would temper for say 20/30 mins minimum at 340deg C no need to quench but if you remove them from the furnace place on a fire brick or wood surface. Remember if you can't file it its too hard !

The distortion you are experiencing may be because you have an assembly rather than an individual piece? I have never experienced this phenomenon.

John