[JayWal]

I thought I'd start this thread for people who have questions or have some myths about the metal we work with that they'd like confirmed or debunked. I'm a self taught hobby welder and im always lookin' to get learned, so here's a couple id like cleared up.

Myth #1 that I've heard so often and I dont believe its true, is I've heard it said that a weld is harder than the base metal. We're talking mild steel only here. The reason I think it's baloney is because I've fixed worn out holes by filling them with weld and simply re drilling, with no issues at all. So can someone smarter than me chime in,if you, say, plug weld a 1/2" dia hole with Er70 wire, be it whatever process, is that plug of weld metal now harder than the base metal around it?

Another question kinda related, is it even possible to harden mild steel? I dont know jack about the different grades but let's say your run of the mill, hot roller 2x1/4" flat bar. It is even possible to harden it? What dictates if it's hardenable or not?

Sorry if this thread is just a rookie yammering, just trying to clear up some questions bouncing around my head.

Anyone else have questions about the metals we weld, post em up and let's see if we can find some answers.

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[Oscar]

The weld may have a higher tensile strength, but not necessarily harder, which is a different property. Carbon content in steel dictates hardenability. Typically need more than 0.3% carbon content. The more C %, the more hardness can be attained. It all has to do with the percentage of % martensite that you able to produce within the microstructure via a sufficiently fast cooling rate. Higher C% can result in more martensite creation. There are other elements that facilitate hardening: Moly reduces the required cooling rates more so than other alloying elements, but chromium and vanadium also play a part, as does nickel.

[cj737]

To your first point, “harder” is an oft misused adjective to describe welds and weldment. Mild steel is a hardness measured typically on the Rockwell scale. Mild steel being fairly soft, Tool steels being much harder.

More commonly, welds are described as “stronger” because you’re rating the tensile strength of the rod (60k, 70k, 80k) to the mild steel (normally about 18-35k).

Yes, mild steel can be hardened through heat treatment. How much harder is based upon the original material. But you can probably get HR flat stock up to 60 Rockwell which will be harder, but much more brittle. Hardening is usually followed by tempering to restore some ductility to avoid shattering the steel under impact. Lots of science, lots of recipes.

You can make hardening very scientific or “DIY” and heat by color and temper afterwards in a grill or oven. It will generally get you by in the Home Shop.

[BugHunter]

Yes, mild steel can be hardened through heat treatment. How much harder is based upon the original material. But you can probably get HR flat stock up to 60 Rockwell which will be harder, but much more brittle. Hardening is usually followed by tempering to restore some ductility to avoid shattering the steel under impact. Lots of science, lots of recipes.

Mild steel is not going to harden in any way shape or form. You can carburize it and surface harden the material but that is not the same thing as heat treating and hardening. That is more a nitride process or you can introduce carbon into the steel when it is hot using something like casenit powder but that's a totally different process. The hard surface will only be about thirty or forty thousandths deep.

[BugHunter]

Myth #1 that I've heard so often and I dont believe its true, is I've heard it said that a weld is harder than the base metal. We're talking mild steel only here. The reason I think it's baloney is because I've fixed worn out holes by filling them with weld and simply re drilling, with no issues at all. So can someone smarter than me chime in,if you, say, plug weld a 1/2" dia hole with Er70 wire, be it whatever process, is that plug of weld metal now harder than the base metal around it?

That is a serious trick question because there are way too many combinations of Base Metal 2 filler in order to make that statement. The answer to this question is really Way Beyond the scope of the Internet. Lol

Another question kinda related, is it even possible to harden mild steel? I dont know jack about the different grades but let's say your run of the mill, hot roller 2x1/4" flat bar. It is even possible to harden it? What dictates if it's hardenable or not?

As I said above, let's just say no. You can't Harden it.

[Poland308]

Welding by definition is a melting of the two base / parent metals and if any filler metal. Your weld is like a soup, it is what you put in it. A weld without filler metal, can in some cases be harder than the surrounding metal if done without filler metal. But this depends entirely on the properties of the base metal, and the process used to do the weld. Best case scenario (this is what welding of standard carbon pipe is supposed to be designed too) is that the weld should be undistinguishable from the surrounding metal. In strength, ductility, corrosion resistance, and (insert any other way you like). When you grind down into a weld it should be the same as the metal you weld to. If you bend it it should bend the same. Think of a bend test, engineered bent tests are used as a way to prove that your weld is the same as the base metal, if it’s harder or defective in some way it will crack or break.

[VA-Sawyer]

Jody likes the book 'Metals, and how to weld them'. He suggests getting it from the Lincoln Foundation. That is what I did. The book is a good starting point for these kind of questions.

[JayWal]

I guess all the people doing repairs like building up a worn shaft with weld and turning it back down, or line welding and boring, filling up holes and turning them down again are proof as well that mild steel won't harden just from welding.

Thats a good point on the bend tests, I didn't think of that.

I do remember Jody recommending that book, ill check it out.

Thanks for the info, im sure I'll be back with some other rando questions [emoji4] I'm from a farming background, so there's a ton of disinformation like that floating around. Mostly from old timers that have never welded with anything besides an ancient tombstone stick welder. Who think welding oilfield pipe through 1/4 of tar and rust is just fine, just "burn through it!" And believe mig is for sheet metal, stick is for the heavy stuff that needs to be "strong". Which the last Welding T&T Podcast showed is a pretty broad term.

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[Poland308]

I like this thread!

[DavidR8]

Definitely reading this one.
I have learned a ton from you good folks.


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[VA-Sawyer]

I used to believe that mild steel could not be hardened. However, it seems that I was wrong about that....

Hardening of steel depends on the Carbon content, AND the rate of cooling. According to pages 137 and 138 of 'Metals and How to Weld Them', steel of .2% Carbon can be hardened to a Rockwell C of 50, by quenching in iced brine. So, mild steel can be hardened if it is cooled quick enough. I did not know that, until looking it up for this thread.

[BugHunter]

I just did a quick search on the subject and while there are links to discussions on "hardening" mild steel, I have yet to find someone who can actually quantify what's possible, and every link also seems to have info about the disadvantages/dangers of doing a water quench. Things like the properties of the steel going to hell and such. Stress cracking, brittleness, etc. And I seriously doubt you could measure the result with an Ames gauge with diamonds on the C scale. Maybe the B scale.

Hey, maybe it's possible. I don't know. I do know that 4130 at the place I buy, is the same price per pound as hot rolled, so if I want steel I can harden, I get steel designed for hardening.

If you want to surface harden, yes, mild steel works splendidly with this stuff below.

[LtBadd]

I'm sure this discussion could be taken to another level by someone with a degree in metallurgy, from a brief search I'd say the term "mild steel" refers to carbon steel with 0.04 to 0.3% carbon.

More info HERE

[sbaker56]

Another point is that "hard" is relative, I'd say compared to aluminum, mild steel is pretty dang hard, compared to properly hardened tool steel? Not even on the same scale, literally. Most filler material has less carbon than the parent metal, so I'd doubt it to be much harder if any at all than the base metal, but the difference is unlikely to be the slightest bit significant in comparison to a file or drill bit.

Likewise, I can tell you it's not hardenable in a traditional sense, it'll never reach anywhere near 60HRC and without a brinelll or rockwell tester you'll likely never notice the difference, however back when I frequented knife forums I had heard that "superquenching", which was some mixture of water and dish soap intended to more rapidly quench the steel than water alone had gotten results on low carbon and otherwise poorly hardening items like rail road spikes.

[Poland308]

Unless it’s a hard surface rod or a wear hardening rod.

[JayWal]

I may have asked that in the past, but what about flame cut edges or plasma cut edges? I've heard guys say they ruined a deburring or countersink bit on plasma cut holes. Can that leave a hardened edge?

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[Oscar]

I may have asked that in the past, but what about flame cut edges or plasma cut edges? I've heard guys say they ruined a deburring or countersink bit on plasma cut holes. Can that leave a hardened edge?

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That is not the same as carbon-induced martensite-production hardening via quenching. That is more nitriding at the surface, which is very shallow.

[JayWal]

I may have asked that in the past, but what about flame cut edges or plasma cut edges? I've heard guys say they ruined a deburring or countersink bit on plasma cut holes. Can that leave a hardened edge?

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That is not the same as carbon-induced martensite-production hardening via quenching. That is more nitriding at the surface, which is very shallow.

No definitely not the same. I think its safe to say now that mild steel can't be hardened beyond surface hardening, at least not in a way that's reliable and repeatable.

Maybe I should've started off by a better definition of "hard". I've never used any hardness guage, I have no idea how hard Rockwell 50, 60, 70, 2,000, is. To simplify it for us hobbyists, I'd call it hard when decent drill bits start to struggle to bite into it. Whatever Rockwell number that would be.

Thats actually what my first question was about. People saying that you can't drill through a weld, it's too hard. I've not encountered that yet.

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[cj737]

Thats actually what my first question was about. People saying that you can't drill through a weld, it's too hard. I've not encountered that yet.

Crappy bits for sure. A decent HSS bit will drill right through mild steel, welds, and even stainless with a bit of cutting fluid. Start small to get a pilot hole, then upsize. Starting with a large bit in a hand drill is making hard work (forgive the pun) of drilling through any steels.

[BugHunter]

I may have asked that in the past, but what about flame cut edges or plasma cut edges? I've heard guys say they ruined a deburring or countersink bit on plasma cut holes. Can that leave a hardened edge?

This is a trick question. The burnt material isn't the same thing as the unburnt. It goes through a change of properties and while the steel isn't too hard to cut, the dirt/soot is. Think of it as "rust", which in a certain sense, it is. Oxide (iron, aluminum, etc) is really hard stuff. The base metal isn't the problem, it's what is on the outside. Alum is very soft, but the alum oxide on the outside of the aluminum is extremely hard. Even if it's really just a shallow layer, it's harder'n the hubs of hell.

No definitely not the same. I think its safe to say now that mild steel can't be hardened beyond surface hardening, at least not in a way that's reliable and repeatable.

Correct.

Maybe I should've started off by a better definition of "hard". I've never used any hardness guage, I have no idea how hard Rockwell 50, 60, 70, 2,000, is. To simplify it for us hobbyists, I'd call it hard when decent drill bits start to struggle to bite into it. Whatever Rockwell number that would be.

There are many scales, each of which are for different types of materials. The same gauge will work on many items just by changing the stylus to one intended for that material. I have a hardness tester, and I have anvils and stylus for B and C. B for instance is what you'd test brass with, copper, aluminum, etc. The anvil is a carbide ball and hardness is determined by how far it sinks into the metal.

C is for harder stuff. That uses a diamond as the test point. Obviously, it sinks in a lot less.

On the C scale, let's take annealed metals, they're generally 28-32 or so. In that range. 4130 or 4140 are real common tool steels. At max hardness (doesn't mean max tensile, max charpy, max ductility, etc) it'll be around 56C. Drawn to a typical usable state, it'll be from say 38-48 (give or take). 38, not fun to drill, but you can drill it. 40, starting to get hard on cheap drills. 45, good luck with almost any cobalt or M2 drill. 50, forget it, you'll need a tool not found in a home.

4340 Steel, even stronger and harder than 4100 series, can approach 60 Rockwell, and has tremendous mechanical properties while still in the mid 50s. It's tough stuff, but also considerably more expensive. At 55, this will put permanent marks in your nice Nicholson flat file.

A2, D2, etc... Max hardness around 67 RC. At 60 it'll scratch glass. easily. Machining is all but out of the question. You can grind it and that's about it.

Only unusual metals go to 70 and above. Rare stuff.

Thats actually what my first question was about. People saying that you can't drill through a weld, it's too hard. I've not encountered that yet.

If you weld a piece of 4140 steel, I seriously doubt you can drill through that.

Another point is that "hard" is relative, I'd say compared to aluminum, mild steel is pretty dang hard, compared to properly hardened tool steel? Not even on the same scale, literally. Most filler material has less carbon than the parent metal, so I'd doubt it to be much harder if any at all than the base metal, but the difference is unlikely to be the slightest bit significant in comparison to a file or drill bit.

Likewise, I can tell you it's not hardenable in a traditional sense, it'll never reach anywhere near 60HRC and without a brinelll or rockwell tester you'll likely never notice the difference, however back when I frequented knife forums I had heard that "superquenching", which was some mixture of water and dish soap intended to more rapidly quench the steel than water alone had gotten results on low carbon and otherwise poorly hardening items like rail road spikes.

I agree with this 100%.

Crappy bits for sure. A decent HSS bit will drill right through mild steel, welds, and even stainless with a bit of cutting fluid. Start small to get a pilot hole, then upsize. Starting with a large bit in a hand drill is making hard work (forgive the pun) of drilling through any steels.

Somewhat true if the material you welded can't be hardened. But if it can harden, a hss drill won't last 10 seconds. Regardless of coolant or spindle speed.

[sbaker56]

Everyone has pretty much covered what I'd have to say on the subject of flame cut/plasma cut edges. the crap in dross is a nasty mixture of oxides some of which can be very hard, and given that air is primarily nitrogen, plasma can nitride cut edges leaving a extremely thin but very hard layer.

A good point on welding hardenable steels by BugHunter, you will get some diffusion of the base and filler metal during a weld which means if you weld 4140 even using ER70s2, the resulting weld might very well be quite hard If you use matching 4140 filler it'll be VERY hard, your bead will get one hell of a quench as the surrounding metal pulls heat away, so you could end up going from a nice soft annealed piece of steel to something that'll skate a file.

Another occasion where you could end up with up with a weld that's much harder on bits and cutters is if you like welding anything you come across with ER312 like I do. It works well for joining just about anything ferrous you'll come across, but it's quite a bit more wear resistant that A36 and harder as well, some people even like it for hard facing so while not nearly as hard as quenched tool steels, it still wouldn't be fun to drill.

[Poland308]

Burning up drill bits is another matter. SS is notorious for burning up drill bits, but it’s softer than mild steel.

[JayWal]

Burning up drill bits is another matter. SS is notorious for burning up drill bits, but it’s softer than mild steel.

Thats true, with the wrong speed vs feed pressure you can burn up a good bit on fairly soft steel.

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[BugHunter]

Burning up drill bits is another matter. SS is notorious for burning up drill bits, but it’s softer than mild steel.

Stainless generates dramatically more friction and heat while being drilled, while being a terrible conductor of heat, thus causing a snowball effect of heat and friction. It just doesn't conduct the heat away from the work area. Then too, SS is notoriously gummy when machining. It tends to smear rather than cut, and it chip-welds something terrible on tools. It might not be hard but it's extremely tough.

[sbaker56]

I have some 1/8" wall stainless tubing that completely destroyed a brand new bi metal Sawzall blade less than 2" into the cut. When I say destroyed I mean the teeth were completely worn to nubs, not simply dull, but nearly gone all together.