Recommend a grade of steel

Only if you divulge the load it is going to see. What is diameter of pump ram and boiler pressure, diameter of pin and length of bearing?

Thanks Duncan. The pump ram is 5/8 inch, boiler pressure 120 psi, pin diameter is 1/4 inch and the bearing length is 3/16 inch.

I have no knowledge of calculating the stresses so please advise. TIA

From the comfort of my armchair, I think Silver Steel was the right choice. Some yield strengths from the Model Engineer's Handbook:

• Aluminium, 5 tons per square inch
• 60:40 Brass, 8 tons per square inch
• Malleable Iron, 8 to 10 tons per square inch
• Dural, 15 to 19 tons per square inch
• Free Cutting Mild Steel, 20 tons per square inch
• Mild Steel, 24 tons per square inch
• Stainless Steel, 36 tons per square inch
• High Tensile Steel, 40 tons per square inch
• Silver Steel, 35 to 50 tons per square inch

Bearing pins are worth hardening if they are subject to high wear AND are difficult to replace.

Unhardened Silver Steel is stronger than most common metals and could be used as a pin without heat treating it. Not the best possible, but pretty good compared with common alternatives.

The performance of Silver Steel can be improved by heat treating it, and this is probably where Robert's pin went wrong. Working with small parts and a blowlamp, it's quite easy to botch the heat-treatment. For best results it's necessary to heat the part to the correct temperature, (cherry red in dim light whatever that is), hold that temperature steady for a time related to the weight of the part, quench the part without allowing the it to cool en-route, and then remove the part at about 200°C and put it straight into a pre-heated oven to slowly anneal it.

Plenty of opportunities to get it wrong:

• Steel not hot enough or too hot for the incorrect time. (Too long or too short)
• Steel cools on the way to the quench
• Insufficient quench liquid (bucket rather than cupful)
• Failing to stir the part if the coolant is water
• Cooling too quickly in the quench causes Silver Steel to become brittle and - in the worst case - to crack. Problem here is microcracks, invisible to the eye, which severely weaken the part, especially if it's repeatedly stressed as a pump pin would be.

My inexpert experience of silver steel suggests it's fairly forgiving of mistakes. However, early on I had a high failure rate - either not hard, or super-brittle. Better now. I think the main difference is not flapping about, instead: smoothly heating up to about the right temperature, holding it roughly for the right length of time, not too hot, and then putting the part straight into the quench and stirring vigorously.

How the part is held can be problematic too. Pliers act as a heat-sink, likely to make one end of a short pin much cooler than the other on the way to the quench. Various ways of avoiding that, for example making the pin much longer than required and grinding it to length at the un-held end.

If the part comes out very hard, it needs to be annealed. Small parts are fairly easy - they can held at 200°C for half an hour in a domestic oven. Might be necessary to get a divorce before this is allowed...

My advice, persist with Silver Steel.

Dave

Edited By SillyOldDuffer on 05/10/2021 10:16:38

Thank you for a very comprehensive reply. Looking at the respective yield strengths, do you consider high tensile steel viable in my circumstances? I'm thinking I could turn down a suitable high tensile bolt and use the pin without heat treatment.

I appreciate that the pump pin is not in tension so maybe shear strength is more important?

Edited By Robert Smith 24 on 05/10/2021 11:36:37

pump area = 0.307 sq.in., so load is 0.307*120=36.8 lbs, add 10% to allow for pressure drop in clacks and friction, say 40.5 lbs. Presuming the pin is in double shear the area is 2*0.25^2*pi/4 = 0.098 sq.in, so the shear stress is 40.5/0.098 = 413 psi, which is quite low. Yield stress of even EN1A is 230 MPa, 33400 psi, and BS2573 would recommend 37% of this (as allowable shear stress, so you are well in. The bearing load is a different matter. The projected area is 3/16*1/4 = 0.047 sq.in, and so the bearing pressure is 864 psi, which is on the high side, so bronze bearing and case hardened pin would be my choice. Good provision for lubrication as well. Avoid notches/sharp edges as much as possible, and if it has a thread on the end avoid case hardening that. There are a couple of ways of doing this. One is to apply the case compound before you cut the thread, allowing it to cool slowly, then cut the thread, then get it red hot and quench, the other (which I've never tried) is to copper plate the bit where the thread is going to be. Cooling a small part slowly is not that easy either, trial and error, if you can still machine it it's ok.

I know that silver steel is the go-to material for model engineers, but in well over 40 years in industry I've never seen it specified for a machine part, except for punches, dies, trigger blocks etc for which a very hard part is needed

Thank you all for your excellent replies. I am giving the matter further thought.