Mitigate strength reduction when welding after heat treatment

[Zeyneb]

Hi there,

I realize I can use some help. Some weld projects are simply trail and error. But this one is where metallurgy play an important role on a expensive car part; the rear axle beam. So this is the rear axle beam on my 905 kg, FWD car. A 1997 Toyota Starlet.


The rear axle is equipped with a panhard rod. I purchased a performance panhard rod but I found out that the bushing on the panhard is wider than the original from Toyota.


This is the mounting point on the rear axle for the panhard. The shoulder needs to be 6mm longer for the wider bushing to fit on. Also the threaded end needs to be 6mm longer, because now the nut only has a few threads engaged. The axle beam is a U shape with a wall thickness of 4mm. At the panhard mounting point the U shape is reinforced with another U shape over it.

The panhard mounting shaft does penetrate through the axle beam and is welded on both ends. I think the section that goes through the axle beam is about 8mm thick. The picture below shows how the rear end of this shaft is welded on the axle beam.

My objective is to grind away this shaft and redesign the shaft with the correct dimensions. But how to make this strong enough to be used on a car? As suspension parts are usually mounted with class 10.9 fasteners, that strength is something to aim for. So at least a yield strength of 1040 N/mm2. Alloys such as 4140 and 4340 must be heat treated in order to reach such a strength. I also know that with welding I will reverse the strength of the heat treatment in that area. But to what kind of depth? Are there ways to mitigate the reduction of the strength during welding? Like selecting a specific alloy for this. I do have a TIG machine and ER80S-D2 filler rods. Would that seem fine for this?

Thank you very much for your attention and time.

[Doctorandus_P]

Any DIY-er with a bit of experience can weld mild steel to make things that are "pretty strong", but once you get beyond that, all bets are off. Nearly all steel qualities stronger then around 360N/mm2 need special treatment when welding, and the exact treatment depends on the alloy used.

Another consideration is, what are the consequences when the weld (or the material next to the weld) fails?
Loosing a broken off rear wheel on the highway is probably not lethal to yourself (unless you overreact and put the whole car into the guide rail), but where is the wheel going?

Welding up some sheet metal or body parts of cars is harmless, but once you get into critical parts such as the axles or any high-strength steels used in a car you are getting near the madness zone. Maybe bring it to some guy in India or Pakistan. Plenty of people (and youtube vid's) there of people claiming to weld crankshafts together.

[langwadt]

make custom nut instead

[jpanhalt]

Ditto.  Counterbore or shorten the bushing to accept a suitably profiled nut (i.e, one that will fit inside the counterbore).  Of course, that recommendation is made without knowing any dimensions.  There would probabby be other ways to get it done, if the new bushing is pretty thin.  But still, that general approach would be preferable to grinding off and rewelding.

Edit: I don't see any hole in that threaded shaft.  Isn't a castellated nut and cotter pin used?

[coppercone2]

I would not touch that shit personally. i would also not trust a electronics forum for this.

For the kind of critical weld lets see.. how do you check for cracks. how do you design the pre weld prep.

the part that people are recommending is probobly a much better idea then to try to weld this.

heat treated stuff likes to crack when you weld it probobly.

I think its probobly a very well engineered part (its low cost mass production) that probobly was not built with too much of a margin to use as little material as they could safely to optimize fuel efficiency and cost. Any reduction in strength from gross heating might probably result in a compromised part. Particularly on a high stress axle location.

this is what it does in a car


I think your weld go bye bye when you hit a bump at moderate speed

I think this is kind of like a DIY femur repair when at most people are maybe stitching up a small wound with their welders.

[mikerj]

think its probobly a very well engineered part (its low cost mass production) that probobly was not built with too much of a margin

This kind of design will have lots of margin built in, it makes zero sense for a manufacturer to shave margins down to the minimum on something cheap to make but with potentially significant consequences if it fails.

However I'd make a sleeve nut as others have suggested, no welding needed and a simple part to make for a machine shop, or if the OP has a lathe.  It also means the axle remains compatible with the correct sized part if it's changed in the future.

[coppercone2]

I am certain the margin goes out the window when you hit a pot hole with that weakened weld. It's a miracle they stay intact with a brand new part. Or one of those extra nasties where the wheel rides up extra high on debris and then falls into the pot hole right behind it, on a cold day. or hitting like a small log segment that rolled off on the road after a tree clearing team forgot about it.

Its over engineered to withstand riding up on the curb by accident at 8 miles an hour

but seriously, heat treated stuff LOVES to crack after its welded. And re-heat treating a weld for strength is also hard.

[JJ_023]

Here are my two cents on the subject matter.  I deal with this type of stuff all the time. 

First of all you can buy a cheap hardness tester and find out if it is indeed hardened. 

Google "Fowler 52-760-000-0"

to get an idea of what I am talking about.  They are fairly accurate for your experiment.

I personally highly doubt it and it is probably just regular mild steel.

If it is 4140 as you assume it is.  ER70S-2 is perfectly fine to use as filler.  You would also need to preheat the area to about 400° F to avoid any weld cracking afterwards.  I would also recommend a gentle post heat.

I would also recommend that you assess your abilities realistically.  I personally would not hesitate to do something like that.  Keep in mind that not only are you risking your safety but also whatever people are around you when you operate that vehicle when you assess your abilities.

[Zeyneb]

Hi JJ_023,

Thanks so much for your advice! I had given up on the possibility that someone here would give me actual advice to pursue on my initial plan. But now your post was a really positive surprise

I was already started with this. I'm motivated to make this a success whatever it takes.

I have already ground away the main tube that is mounted inside the U shape. I had cut it in the middle to be able to push the ends inside. Like this:



That tube is an inch (25.4mm) in diameter with a wall thickness of 3mm. Shall I do 4130 tube as a replacement? I assume this should not be heat treated, right?

Then I've also removed that mounting stud on the 2nd photo of my initial post. I do think I'm going to buy that Fowler hardness tester, like you suggested and have a local watchmaker help me judge the results of that experiment.

If it is indeed not heat treated. What would be a good alloy steel to have it possibly stronger that what Toyota made? But also as you know meet the toughness and ductility requirements.

Sure I'm going to preheat. For this I'm thinking about a IR thermometer to purchase. One with the emissivity of a surface to be able to adjust. I will determine the emissivity with a comparison with a thermocouple measurement. Do you know if an IR thermometer works well on a sandblasted surface?

[coppercone2]

probobly should use temperature test pens of the correct type, you can't fool melting point

[JJ_023]

Hi JJ_023,

Thanks so much for your advice! I had given up on the possibility that someone here would give me actual advice to pursue on my initial plan. But now your post was a really positive surprise

I was already started with this. I'm motivated to make this a success whatever it takes.

I have already ground away the main tube that is mounted inside the U shape. I had cut it in the middle to be able to push the ends inside. Like this:

(Attachment Link)

That tube is an inch (25.4mm) in diameter with a wall thickness of 3mm. Shall I do 4130 tube as a replacement? I assume this should not be heat treated, right?

Then I've also removed that mounting stud on the 2nd photo of my initial post. I do think I'm going to buy that Fowler hardness tester, like you suggested and have a local watchmaker help me judge the results of that experiment.

If it is indeed not heat treated. What would be a good alloy steel to have it possibly stronger that what Toyota made? But also as you know meet the toughness and ductility requirements.

Sure I'm going to preheat. For this I'm thinking about a IR thermometer to purchase. One with the emissivity of a surface to be able to adjust. I will determine the emissivity with a comparison with a thermocouple measurement. Do you know if an IR thermometer works well on a sandblasted surface?

Let me help you by addressing some of your points.

I don't know why you are fixated on 4130.  It is most likely mild steel.  But nor I or you are sure at this moment so you need to figure that out first.  You can call around to scrap metal places and see if they have a machine that will analyze the metal for you.  A large outfit should have a handheld XRF Analyzer.

You can also do a spark test which won't be as reliable.

https://electricmotorglider.com/wp-content/uploads/2015/12/Spark-Test-web.pdf

You should replace the tubing with what the original tubing was because that is what was originally engineered for that vehicle.  If you're looking for more strength there are other options but unless you redo the whole part you will have a failure somewhere else.   

Besides 4130 you can also look at DOM tubing.

The pre-and post-heat recommendations don't have to be that specific.  As mentioned by the poster above a temp stick would be probably your best go to method.  It would be a lot more accurate than any of your other ideas.  Just Google "Markal Thermomelt Temperature Indicator".

The hardness tester I recommended was just a reference you can get any brand you like as long as it's reputable.   They are super easy to use.  It's basically a file you rub on the metal.  If it cuts the metal the hardness is below that file.  If the hardness is above the file it will just skate on the metal.

I think it is great that you are attempting this but as I stated earlier know your limitations and be honest with yourself don't jeopardize your safety or the safety of others,

[Zeyneb]

Hi JJ_023,

Thanks again! That XRF analysis seems to be a really good idea. But considering the rough and know-it-all attitudes from scrappers in my country I doubt if that is the best place to go to. But then a laboratory that offers XRF analysis might be prohibitive expensive. I can however ship the samples abroad to have them analyzed by careful and patient workers elsewhere. Maybe in eastern Europe or so.

Will an XRF analysis just provide the chemical makeup of the steel? It will not tell anything about the hardness, does it? I should do that test separately.

You should replace the tubing with what the original tubing was because that is what was originally engineered for that vehicle.  If you're looking for more strength there are other options but unless you redo the whole part you will have a failure somewhere else.   

Now when I think about this, that might be a really good point. If I had disregarded this I might have chosen a tube that is too stiff compared to the original and that would put too much stress on the U shape and it would fail there in some driving condition. Would that be a valid example of a scenario what you meant?

I should point out that I do not plan to do any racing or off-road stuff with the car. Can you think of an some other limitation that I might overlook at this moment? One thing I do think of is to make a nice interference fit for the panhard mounting shaft. This to have the forces primarily transfer through the interference fit and secondary through the weld. Would that be a good idea?

[JJ_023]

Hi JJ_023,

Thanks again! That XRF analysis seems to be a really good idea. But considering the rough and know-it-all attitudes from scrappers in my country I doubt if that is the best place to go to. But then a laboratory that offers XRF analysis might be prohibitive expensive. I can however ship the samples abroad to have them analyzed by careful and patient workers elsewhere. Maybe in eastern Europe or so.

Will an XRF analysis just provide the chemical makeup of the steel? It will not tell anything about the hardness, does it? I should do that test separately.

You should replace the tubing with what the original tubing was because that is what was originally engineered for that vehicle.  If you're looking for more strength there are other options but unless you redo the whole part you will have a failure somewhere else.   

Now when I think about this, that might be a really good point. If I had disregarded this I might have chosen a tube that is too stiff compared to the original and that would put too much stress on the U shape and it would fail there in some driving condition. Would that be a valid example of a scenario what you meant?

I should point out I do not to plan to do any racing or off-road stuff with the car. Can you think of an some other limitation that I might overlook at this moment? One thing I do think of is to make a nice interference fit for the panhard mounting shaft. This to have the forces primarily transfer through the interference fit and secondary through the weld. Would that be a good idea?

I think going to the scrap yard is probably the easiest way.  If they have the machine and you seem like a decent human being they will probably do it for free.  If not throw some money and they will do it and it would be a lot cheaper than going to a lab.  I could be completely wrong on all of this as you are in a different part of the world.  So what I tell you is from my perspective.

If you want to know the hardness you can do a test with those files I recommended or pay money for somebody with the proper equipment to tell you (probably the cost of those hardness files).  If you have a lot of experience with metal you don't even need the special files you could tell with a regular file (within reason).  Your not going to be able to specify the hardness but you would be able to tell if it's soft on hardened.

If you think you have the capabilities to make an interference fit then you should have the capability to make the perfect fit.  The material contracting from welding is going to create an interference fit anyway.  If you start with an interference fit your just going to make it more complicated and difficult to mount the part.  Plus how are you going to bore the hole, I am not saying it cannot be done but that is a big weldment.

My advice to you is stop trying to figure out all the different possibilities.  Find out what you're dealing with first in terms of composition of the metal.

[Zeyneb]

If you think you have the capabilities to make an interference fit then you should have the capability to make the perfect fit.  The material contracting from welding is going to create an interference fit anyway.  If you start with an interference fit your just going to make it more complicated and difficult to mount the part.  Plus how are you going to bore the hole, I am not saying it cannot be done but that is a big weldment.

Bore the holes? Regarding the panhard mounting point which is at a slight slant, I made a jig with some wood and some bar clamps to have a piece of steel tube mounted coaxially as a guide. I can buy some mild steel round bar to put inside that steel tube of the previous sentence and have a machine shop drill properly sized hole on a lathe to have a guide for the shaft diameter of a reamer.

And regarding the main beam tube. I plan to first spot weld some steel tube temporarily to use as a guide of a big reamer. Maybe I buy that reamer from our Chinese friends at Aliexpress to safe money.

Once the holes are good I know what to specify to the machine shop for the shafts.

My advice to you is stop trying to figure out all the different possibilities.  Find out what you're dealing with first in terms of composition of the metal.

Alright I will do that. I hope you will be around in the future so I can ask something else. Thanks a lot for your advice!

[JJ_023]

If you think you have the capabilities to make an interference fit then you should have the capability to make the perfect fit.  The material contracting from welding is going to create an interference fit anyway.  If you start with an interference fit your just going to make it more complicated and difficult to mount the part.  Plus how are you going to bore the hole, I am not saying it cannot be done but that is a big weldment.

Bore the holes? Regarding the panhard mounting point which is at a slight slant, I made a jig with some wood and some bar clamps to have a piece of steel tube mounted coaxially as a guide. I can buy some mild steel round bar to put inside that steel tube of the previous sentence and have a machine shop drill properly sized hole on a lathe to have a guide for the shaft diameter of a reamer.

And regarding the main beam tube. I plan to first spot weld some steel tube temporarily to use as a guide of a big reamer. Maybe I buy that reamer from our Chinese friends at Aliexpress to safe money.

Once the holes are good I know what to specify to the machine shop for the shafts.

My advice to you is stop trying to figure out all the different possibilities.  Find out what you're dealing with first in terms of composition of the metal.

Alright I will do that. I hope you will be around in the future so I can ask something else. Thanks a lot for your advice!

A reamer that size is going to be pretty expensive.  Have you thought about actually taking it to a machine shop?  They can answer a lot of your questions.  They definitely can make the shaft and they probably can bore the hole for whatever fit you want.  You can dictate to them what your preferences are as well as directing the whole project in the direction you want. 

Some things that you are trying to do require specialized tools.  Unless you are going to be setting up a machine shop sometimes it just makes sense to pay somebody to do something even if you know how to do it.

Having said that.  If my original assumptions are correct about it being mild steel.  You can probably work the hole to the right dimension with a good file and a standard as reference.  Remember you are welding this, this is not some dowel pin hole that you are using to locate a fixture.