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rsterne · Moderator

Can anybody clue me in as to the differences in these two Chrome-Moly steels?.... Strength?.... Machinability?.... What has been your experience?....

Bob

kanyon · Silver Status Member

jlwilliams · Veteran Member

4140 and 4130 are almost identical, except that 4140 has .40% carbon and 4130 has .30%. For practical purposes that just means the 4140 hardens up a little harder than the 4130 when you heat treat it.

Both can be worked in the 'pre hardened' condition and both are often sold as either pre-hardened or full anealed (dead soft). In general, if you can just machine it and use it, go for pre-hardened. If you are going to weld it, which would mess with the hardness in the effected area, then buy it anealed and heat treat it after you do what you have to do.

you can use them almost interchangeably. Good general purpose steel to use on parts that require more toughness than hardness. Stronger than mild steel, softer than tool steel.

Hope that helps.

rsterne · Moderator

Thanks.... you helped a lot....

Bob

Alstone · Moderator & Site Supporter

Basically the Tensile Strength of 4130 is less than 4140, also the hardness, but the impact strength is better.

Al

rsterne · Moderator

Al....

In the state you purchase them (ie tubing).... is there any significant difference in the machinability?.... I don't have the facilities to do heat treating or annealing.... Why this has come up is I need some 4130 to use as the main tube in a PCP and I have been offered some 4140.... How does it turn / drill / mill compared to mild steel?....

Bob

Alstone · Moderator & Site Supporter

Bob have a look at the spec's for both steels, I don't think you will have any trouble with 4140 in fact I think it would be better for what you require. As for machinability it is tougher than mild steel but with a good cutting lube you should have no problems, you may need to drop your cutting speed down a bit.

http://www.suppliersonline.com/propertypages/4130.asp#General

http://www.metalsuppliersonline.com/research/property/metals/25.asp

Al

jlwilliams · Veteran Member

IMO, the slight increase in hardness over mild steel kind of helps machinability. You just get a better cut. Soft mild steel sometimes wants to 'mush' in front of the tool instead of cutting a clean chip. Just my opinion, your milage may vary.

i8dirt · New Member

Hello Bob,

Huge fan of your work with the crosmans, PCPs, and related investigations...

My 2 cents worth of opinion, chances are you know this stuff, but for the benefit of those that don't:

see http://www.aircraftspruce.ca/catalog/mepages/4130tubing_un1.php
Direct quote from Aircraft Spruce (they have Canadian distribution, sell by the foot):

4130 This chromium-molybdenum alloy is one of the most widely used aircraft steels because of its combination of weldability, ease of fabrication and mild hardenability. In relatively thin sections, it may be heat treated to high strength levels. In the normalized condition it has adequate strength for many applications. It may be nitrided for resistance to wear and abrasion.

4140 This chromium-molybdenum alloy is a deep hardening steel used where strength and impact toughness are required. It has high fatigue strength making it suitable for critical stressed applications. It may be nitrided for increased resistance to wear and abrasion.

If you are going to build a pressure vessel from this stuff and are going for max strength, there is a few things to keep in mind.

RE machining: I like it better than regular steel, less gummy. Carbide is nice, but a sharp HSS bit works good also. Cutting oil helps and a slower speed. Tubing usually comes normalized, so "easy" to work with.

I would not harden the vessel just to maintain a degree of ductility and safety associated with being a bit "flexible" or in other words, less likely to exhibit brittle failure from some stress riser or bad hardening job.

*With a TIG never weld cold tubing, get it up to at least 20C
*Use good welding rods like an ER70 or ER80, they are of very good composition and don't cause porosity problems like those cheap rods do. If you are not pushing the stress envelope, stainless filler rod is nice to use, it flows and wets easier.
* design your end caps so that the weld prep (open groove) allows for full penetration weld. What is nice is if your end cap has a thin lip that inserts into the ID of the tube that also supports the weld puddle so less likely to blowing a hole into ID whilst welding. Multi pass welds are OK.
*NEVER NEVER NEVER water quench after welding. You can instantly cause microscopic cracking and stress issues. Always allow a slow cool back to room temp.
* Don't overheat your weld because you start getting oxide and porosity issues. You may want to let the job cool back down before doing another weld pass.
* If your electrode touches the weld puddle, stop. Grind out the immediate vicinity so that any tungsten inclusion does not cause a stress riser.
* (optional) If you are real paranoid of your weld, anneal the joint with a torch (non oxidizing flame), dull red should be enough, slow cool.
*do a practice weld or two.
* read up on the net about more do's-do nots.

Silver soldering is also a good option with a well designed joint. An end plug with a tapered, generous lap joint should work. Some of the old solders have cadmium so good venting is needed. No overheating, no quenching, no jaywalking, no shirt, no service etc... Clean joints, about .002-.003" radial gap should be about OK. Read up on prep and gaps for more critical applications.

TRY to pressure test your welds. Fill your tank with non cumbustable fluid like water and then pump with air. This is not a perfect method, but as long as 99.9% of the of the internal volume is not air there should not be a catastrophic explosion if there is a flaw. Of course dry it out well (bake it for a few hours) after testing. Or pressure test with air but have the vessel deep in a barrel of water, so if it blows the water disapates the energy. I would test to 4X working pressure if it was my own job if I could.

Good luck,
Mark