Solve a few concerns hardening 4140 parts

[fastline]

We machine some parts from 4140A. For wear resistance, they need a harder face. Obviously there are probably better materials and processes for face hardening but these parts are less than 1" cubed so hard to not full harden then anyway. We originally made these from 4140HT which is HRC 28-32. That is really not hard enough but we were trying to avoid the process. We found that we needed the Annealed condition for easier machining and further have been hardening them with a torch and oil. each is carefully heated to just when they glow a bright orange. Th process has been getting them to about HRC 50 which is about spot on.

It is my understanding that 4140 can acheive HRC55 but we really don't need it. Our target is HRC48-53. On our first batch, the parts were tempered back at 400F. On all other runs, we have not tempered because of some data we found that indicated that there could be structural concerns if you don't temper back far enough and better to just leave un tempered. We have destructively tested the parts and are working great.

However, we want to refine the process and get better. We have a kiln and I am looking to convert it with digital control for now to try it out. However, the largest problem we have that is hurting us on these is the carbon scale that forms on the parts. It does not come off in the tumber and requires that we bead blast each part which is a huge pain. We really need a clean part from HT. IIRC, some propane or inert gas injection in an oven to remove the oxygen. Propane makes me nervous though.

[HWooldridge]

There are inert gas HT ovens on the market - look at "Blue M" ovens and furnaces on the net for ideas. If the job is big enough to support the $, you might want to look at used furnaces. Otherwise, consider using stainless steel foil envelopes to reduce scaling. A bit of paper placed inside the pouch along with the part will consume any trapped oxygen then you drop the whole package in quench.

[fastline]

I could possibly make an inert gas system work for me but I am curious if, just in the time from oven to oil, if that is enough air exposure to scale the parts? I seem to remember some ovens that try to keep gas around the parts right to the quench oil.

I think the SS wrap might be out simply because of time. 200 parts could take a while to wrap and I really need to throw and go. Obviously outsourcing is an option but I am trying to use this as an excuse to get better setup for this. For times when we only need a few parts done and done VERY quickly, it could be helpful to have heat treat ability right in the shop.

I do not plan to make a kiln my permanent oven, I just want to get off the ground with this before making the investment.

I was reading where there are certain inert gas blends uses specifically for this? Totally inert and safe? Well, one indicated there was CO1 in that blend which should be vented I guess.

[HWooldridge]

Using inert gas and controlling the atmosphere with fuel blends are part of the process for different materials. For example, case hardening 12L14 requires an excess fuel atmosphere and introduction of ammonia. You can easily introduce argon into a kiln but you'd have to figure out flow rates and whether that would be cost effective for your application. Commercial ovens are either vacuum or inert gas and it probably would be cheaper to outsource. Depending on where you are located, check out Bodycote Thermal Processing. They have plants all over the US and are usually competitive because they batch process your parts with other customers to spread out the costs.

BTW, you can put more than one part in a stainless pouch.

[Rich Hale]

Knife supply houises carry a product called turco, i use it for heat treating blades. you dip in and then hang it til the part is dry. into my gas forge til it reaches proper temp and then into the preheated oil called for..Then into the oven for correct temper. Scale is almost non existant. usually a wipe with atowel is all that is needed. As these blades willbe mirror finished the get abrasives anyway but no scale pitting. not sure if it will work for you. i get mine from Knife and Gun finishing supply in AZ.

[ThomasPowers]

What sort of through put are you looking at? 200 a year or 200 an hour?

I would look into an induction unit with inert atmosphere.

[fastline]

I think part of the issue is batch size and annual qtys. Only about 400/yr. We typically run in 100qty batches but only because we were nervous about tolerances and scale. It seems the parts are not moving at all and dimensions don't change being such a small part. I suppose raising qty to 200 and outsourcing might be cheaper but again, I am also looking at this as consideration for future work as well. NOT that we want to be a HT company, but only for smaller runs that cost too much to have done. Each part weighs less than 2oz and will fit in a 1" cube.

I guess I did not think about bulk packing the parts in SS bags. I guess the only deal there is no visual inspection before quench. I have been going strictly off of color. I will certainly look into the Turco as well if that can work for us. Dip all the parts before treating. I think our local company wants about 120 for a batch of parts. Not way off there but it would be nice to know we can work with parts in house too.

[Frosty]

RR rail is induction hardened to a few thousandths on the tire contact surface only. You might contact a rail manufacturer and find out what the process is or maybe vendors in your area.

Frosty The Lucky.

[Steve Sells]

salt baths give a clean product, but I worry about why you were told not to temper after hardening? Even a soak at 350F is going to relax problems with brittleness and helps with retained austinite, it can bite you later. Have you tested any a week or so after the hardening with out tempering?

[fastline]

No, the parts that were tested were within 48hrs of heat treat. I certainly hope we don't have issues but would also like to reimplement a temper or look more into that issue. I cannot now remember but there was basically a brittleness graph based on temper and there was an obvious dip in the HRC50 area that indicated we might do more harm than good. I also found several people having success with other parts ar full hard and no temper. I would be open and interested to discuss that matter though.

I am not familiar with the salt bath technique at all. IIRC, this was a brine bath that must tay at temp all the time? IE, not an avenue for only occational HT processes?

[JNewman]

I cannot believe you are considering setting up heat treating for twice a year to save $120/batch. A shop using a vacuum heat treating furnace will treat all your parts scale free. You can't be heat treating the parts with a torch and then cleaning the scale off in less than 2 hours.

[Steve Sells]

I was talking about liquid salts , not a brine; and it is at temp all the time, turns to rock when cooled off. If this is a limited run, I suggest you hIre it out, that would be the cost effective way to go.

[fastline]

Again, we are using this one job as a stepping stone for future projects. I certainly would not consider a HT setup solely for two processes per year.

I am curious though, regarding a vac furnace, surely the parts are exposed to air when transfered to the quench oil? Obviously vacuum seems smarter and safer than inert gas injection.