Thread: heat treating stock axles

Originally Posted by Psimpson7

lol... must be pretty much solid steel to get the other 400kg's

Interestingly going back 15 years, I did break plenty of 10 spline bits and peices in my old 68 s2a.

Well there is the bullbar (brush guard) made from steel flatbar, all the steel attachments (steps, lifting rings, etc, etc), raised suspension mounts...

Then the 50 layers of olive drab applied when they couldn't find any more rocks for the grunts to paint

it still gets me,
why when someone pipes up and says he has a good idea [in this thread, i think we all agree that it wasn't] we have our say and then we all fall on our swords bagging the **** out of our own chosen marque.
why is that.
it is simple i thought,
the rover product is fine in stock condition, and driven sensibly.
that isn't any different to a nissan or jeep or toyota.
so why get all uptight about how you can break something when you modify it past it's design limit?
grow up.

In solids, the different crystalline and noncrystalline aggregates of atoms and molecules are called phases.

Steel is a multiphase solid and the different phases in steel include ferrite, pearlite, cementite, bainite, austenite and martensite.

Carbon is much more soluble in austenite.

When steel is heated below its melting point it undergoes phase changes (called transformation). The transformation depends on carbon content and temperature, but typically the mix of ferrite plus cementite transform to austenite above 723C (if carbon percentage is high it will transform to austenite plus cementite).

If it then cooled slowly (normalised) the austenite will transform back to ferrite plus cementite.

Bainite may form when austenite is quenched to (usually) between 200 and 400C and held there.

Martensite forms when austenite is quenched rapidly to a lower temperature than when bainite forms. The amount of austenite that transforms to martensite is determined by the temperature - the lower the temperature, the more martensite that is formed.

Martensite is extremely hard and brittle. It is the strongest form of steel.

Quenching to obtain martensite is difficult with thick sections, because it is difficult to reduce the temperature further from the surface. If large sections are quenched too rapidly, suface cracking can occur.

The purpose of adding hardenability improving alloys to steel is to make it possible to produce martensite in thicker sections without quenching too rapidly that surface cracks form.

During quenching some autenite is retained (not transformed to martensite). Cryogenic treatment, after heat treatment can transform some of the retained autenite to martensite - thus improving strength of the article.

The brittleness of martensite is reduced by tempering - the steel is reheated to the tempering temperature, held for the required time, before cooling again. Tempering reduces the hardness somewhat, but greatly reduces brittleness. The higher the tempering temperature, the lower the harness, and brittleness (and vice versa).

Strength of steel is directly related to hardness.

The heat treater needs to know what alloy elements and their percentages are in the steel to determine how to harden and temper it. This affects how fast it is heated, to what temperature, how long it has to be held at that temperature, how to quench it, etc.

In my experience axles don't fail because they are loaded to their static breaking strength.

Damage occurs at lower loads and is cumulative over the lifespan. The cumulative damage depends upon the loading spectum (engineers use Miner's Rule for cumulative damage). This is why axles break at times when the load is not particularly high.

The other common cause of failure is impact loads, but I won't go into this topic at this time.

I wouldn't got to the trouble/expense of heat treating an old axle in an attempt to get a gain in strength.

I have no idea what heat treatment was carried out at manufacture of the stock axles - not something I have been interested in, but if I had to guess, I would say they were probably induction hardened. This hardens the axle for a certain depth and leaves the surface hard and the core tough. It is also a quick and cheap process for a production line.

It is appropriate for axles because the torsional stresses increase from zero at the centre of the axle and increase to a maximum at the surface (similar to how the hardness would vary).

Through hardening may not significantly increase the overall strength.

Originally Posted by harry

it still gets me,
why when someone pipes up and says he has a good idea [in this thread, i think we all agree that it wasn't] we have our say and then we all fall on our swords bagging the **** out of our own chosen marque.
why is that.
it is simple i thought,
the rover product is fine in stock condition, and driven sensibly.
that isn't any different to a nissan or jeep or toyota.
so why get all uptight about how you can break something when you modify it past it's design limit?
grow up.

??? I am not sure who needs to grow up here. Every "marque" has its problems, I think it is important to let people know, rather than deny it exists.

Nissans tend to shear wheel studs - see my other post about a nissan losing a wheel on the highway today. I posted it on another forum and 2 pages of people jumped on and said they knew of the same...

So while Nissan have issues with wheel studs and the ZD30 engine, They got the axle side right with axles up to 1.47" diameter.

Land Rover on the other hand had serious axle breakage problems with the IIA LWB, so it upgraded the axles on the next model SIII, however kept the same axles on the others. When they finally upgraded, they skimped on either the metalurgy or the heat treatment, so the new axles were no stronger. They are STILL smaller than the axles on all comparable vehicles.

So while Land Rover have gotten most things right, the axles have been consistently underengineered on most models, even for stock tyre sizes.

C'mon Ben, I'd much prefer to pretend LandRover ownership is living autopia - lets not get all realistic about these things and spoil it.

In almost fifty years of driving Landrovers my total is one half axle (in 1968, 2a 109 diesel) and one diff - not CW (in 1996 2a 109). But from what I have seen with others I have concluded that axle quality has varied quite a bit, with probably the worst produced in the late sixties.

I also am very dubious about improved heat treatment being a solution. I would guess that most axle failures stem not from poor heat treatment but from defects in the steel or probably more commonly surface defects which serve as stress concentrators, all of these coming from poor quality control ("buy your parts from the cheapest supplier!"*). Obviously, different heat treatment is not going to change these.

John

*Leyland motto

Originally Posted by JDNSW

I also am very dubious about improved heat treatment being a solution. I would guess that most axle failures stem not from poor heat treatment but from defects in the steel or probably more commonly surface defects which serve as stress concentrators, all of these coming from poor quality control ("buy your parts from the cheapest supplier!"*). Obviously, different heat treatment is not going to change these.

John

*Leyland motto

Valiants and others with exposed torsion bars used to suffer breakages of the loaded bar almost always from a surface defect such as a rust pit or the marks left by the jaws of vice grips etc when clamped on and struck to slide the bar out of the housings. The break used to be a spiral, interesting the first time.

Originally Posted by Brian Hjelm

... The break used to be a spiral, interesting the first time.

Spiral break is classical of torsional shear overload failure.

Torsion creates shear stress with the plane of major principle stress (with stress there is are major and minor pricinple stresses at 90 degrees to each other) is inclined at 45 degrees to the rotational axis.

If an axle/half shaft failed due to being statically loaded beyond its ultimate strength it would nearly always fail on this angle.