Heat treatment for Stabilizer bar 1

[Neubaten]

Hello everyone!!

This is my first post here, so I´ll try to be as clear as I can...


Well, I´ve just finished my studies and I´m currently working at a factory that makes parts for automotive industry.

I´m involved in the development of a stabilizer bar for the front suspension set of a certain type of car (volkswagen), which is made from a steel tube.

When I entered the company, the main problem at the development of the product was in the fatigue test. The part got heated during the test and broke at a very little number of cycles, like 6.000 or so, quite far from the specification(50.000 cycles).

I overlooked the prototype process and I found this:

-The tube was badly quenched. The resulting hardness was very low and not uniform throughout the tube (20-30) HRC more or less. The quenchant is water+polymer.

-The tube was not tempered after the quench.

-The tube is 0,35 C, 1,25 Mn in composition, with no other interesting alloyants.

-The fatigue test on the tube is combined torsion/flexion.

Also the tube is shot peened after the heat treatment.


My first steps were to properly set the quench and stablish a temper. Then apply the shot peening.

For certain reasons the quench only holds austenizing temperatures for 40 seconds aproximately, and the temper is made at 300º C and a holding time of 60 minutes.

The tube gets 54 HRC after quenching (which is consistent with the 100% martensite/0,35 C) and 45-46 HRC after tempering.

This is th process that gives the best results in the fatigue test without shot peening. (28.000 cycles)

We reach the target with this process, we even reach at 79.000 cycles.

The problem is that, for serial manufacturing, the big bosses are concerned about the tempering part, as it is a process that lasts for an hour.

I have stated that tempering after quench is necessary for relieving of internal stresses, and the adquisition of certain ductility to ensure the blunting of the tip of the fatigue crack, but I´m not quite sure, as I´m only an insecure newbie.

The bars with tempering gave better results than the bars just quenched, before shot-peening, but maybe just quenching and shot peened would be valid for passing the fatigue test.

Other companies make it quenched & tempered, as the benchmarking papers show.

Please, someone could offer some advice, as I don´t want to screw things up and make my company buy a furnace for an incessary tempering process, which is also very slow...

Also, I´d like to know what´s your take on the "only 40 seconds of austenizing holding time" matter.

Very grateful to you,

Neubaten.

 

[CoryPad]

You definitely need to have the tempering process. It is unprofessional, unsafe, and quite frightening to hear that an automotive supplier doesn't understand the basics of its products, especially ones that are safety-critical like suspension components. Doesn't VW give you a complete materials specification for the part listed on their part drawing? This specification will include requirements like chemical composition, inclusion content, decarburization, heat treatment, etc.

The time for austenitizing depends upon the part dimensions and the prior microstructure, but it can take less than 1 second for transformation to austenite. For example, induction hardening relies upon short time heat treatment times.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

I agree with Cory that the tempering is absolutely required, and that the time at austenitizing temperature does not necessarily need to be very long. Can you provide some additional details on the type of furnace used for austenitizing, the furnace temperature, how the parts are conveyed, etc.? With regards to tempering, there are furnaces that are frequently used for tempering springs, shafts, etc. that have cycle times closer to 10 minutes than 60 minutes. Use the following links for more information:

http://www.pyromaitre.com/eng/home.html

http://www.pyromaitre.com/eng/pdf/rapid_tempering.pdf

 

[dbooker630]

Please provide type of heat, furnace or resistance heat. Also the carbon and manganese levels suggest there may be some boron in the chemistry also.

If you are resistance heating and quenching you could investigate resistance tempering also. Even if furnace heated you may want to investigate transferring an as-quenched tube to a resistance fixture for tempering.

The only drawback is that you typically have some distances on the end outside of the contacts that will not be tempered. Of course if the ends do not require martensitic structure this could work.

The 40 second figure is not unreasonable for resistance heat.

 

[Neubaten]

Wow, thanks!!


You´ve answered quite fast!!


Cory, my company is only developing this project. We are currently supplying VW on other products (all suspension related), and we´ve been suppliers for nearly 20 years, but not sta bars. This is a new product for us, and we are still engineering the processes and the product, and making prototypes.

VW specifies for this product a list of things, and I´m sure I´ve read and re-read them all, and so my work mates. They do not specify any kind of steel, nor the heat treatment or the chemical composition. As long as you get the required spring-back and fatigue life, I think the supplier has the freedom to stablish his own processes and steel grade, even though they all should always remain into the VW normative.


I understand the reason why tempering is necessary and I´m the one who has successfully stablished the quench & temper into the product they were developing, but it´s hard to explain to a big boss that only sees money and time through his eyes, specially when you are not very experienced and insecure.

I would thank a quick, concise answer to the question "Why is tempering necessary for a stabilizer bar?" to put it in front of my superiors. I can´t find one without recurring to long explanations!!


TVP, so many thanks for this useful info, I didn´t know anything about rapid tempering, and I´m going to study it from every angle. This could be a great help for me.

Have you got any info on industrial furnaces that are commonly used for the quenching of stabilizer bars?


Thanks again, mates, I surely was in need of an experienced helping hand.

 

[sreid]

For your management;

The heat and quench hardens and strengthens the steel but makes it brittle. Brittle steel is subject to early fatigue failure. Tempering the steel removes the brittleness with little effect on hardness and strength.

 

[Neubaten]

dbooker, you are correct. We are currently using for prototypes the resistance heating. Also, the steel has Boron, but at half the percentage that the regular 35MnB5 steel.

Problem is we don´t have the controls in the machine to maintain a certain temperature (we have requested them), so we can´t temper.

The temperature can only ramp up, and while this can seem weird, the results we have for as-quenched are good. I think we have a very good, more than acceptable, martensite formation throughout the section. I´ve personally observed it with etchant method, and hardness measurements, all 54 HRC in different points of the section. The tube has a wall thickness of 3,5 mm, by the way.


Would you think we could reduce tempering time using the resistance tempering?? Is it the same theory under those rapid tempering furnaces mentioned above??


And, yes, the ends do not require martensitic structure. Resistance heating is perfectly aplicable, I think. Problem is tempering. I´d give my right arm for any insightful info on resistance tempering.

 

[dbooker630]

Neubaten,
I believe you can reduce total tempering time using resistance as most of the energy is going to heating the part and not the atmosphere. However I would not eliminate or reduce furnace temper without supporting test results. You may also have to adjust your bend program to account for fit changes with the modified thermal treatment.

Many years ago when I first got involved with resistance technology (same application) the furnace temper stayed put and there were issues with keeping the infrared sensor eyes clean from the polymer mist burn-off. The dirty sensors would lead to overtempered parts.

I would contact a temperature control company such as Ircon, Raytek, or Land to see what type of system they can recommend for you.

 

[CoryPad]

Neubaten,

I am not trying to be overly critical of you, but when you say:

They do not specify any kind of steel, nor the heat treatment or the chemical composition.

I don't believe you. This is not standard practice at any automobile manufacturer. Maybe this is a prototype part that doesn't have a fully developed drawing, materials specification, test specification, quality plan, etc. But, a part for series production will have all of these.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Neubaten]

Dbooker, we´ve send the resistance machine manufacturer a request for a set of pyrometers and the necessary electronic control to manage a constant T. Your observation about the optical is quite interesting. It´s true our parts come from the quench completely full of polymer "glued" to the surface.

Anyway, we´ll be ok for prototyping, I think, because we can clean the zone the lens is lecturing before temper.

Looking for series production, I´m aware that we could need some other solution, maybe based on direct contact??.

Thanks for your help, mate.

Sreid, that explanation could suffice, but I´m working more into "the other big and great companies do it, so if we don´t, we are not playing in their league" kind of reasonment, which from my little experience works quite better than standar scientific logic into managers minds.

 

[Neubaten]

CoryPad, I have to admit I took it for granted, but you were absolutely right. I have just read the drawing and tables again with a senior engineer of my department next to me, and we´ve seen that they ask for a certain range of Mn alloys, 0.28 to 0.35 in C content, and the treatment of quench&temper is suggested, albeit they allow others if you are over-specifications.

For me, there wasn´t any other point of view apart from the 34Mn5 steel and quench&temper, which apart from the specification, I was sure it was the safest/most proper/easier.

28Mn4 is suggested with less hardness specification, but I feel 34Mn5 would be easier to handle (from the heat treatment vs. results point of view), and easier to get the requirements, as the 28Mn4 would get less hardness after quench, so you´ve to be careful with temper to not drop hardness excessively, while 34Mn5 can offer a bigger range of tempering without dropping hardness too much.

But, anyway, what do you think they both would comparatively behave from the fatigue side, after heat treatment??

 

[dbooker630]

If there is some range allowed with strength and structure requirements, have you considered a cold bend bar? Perhaps you can develop enough stiffness in the bends. Your boron grade is a good candidate to try out if you can.

 

[Neubaten]

Bar is cold bended, it has some "complicated" bends inside the torsional section, apart from the bends at the ends. Let me see if you can figure it out with this "sketch":


\ /
\ ____ /
\_____/ \______________/


The tube is bended before heat treatment, so my consideration is that the effect of cold bending is lost after heat treatment and shot peening, be it possitive or negative.

Also cold bending, with the resources we have by now, produces certain inevitable ovalization of the tube section, which I´m not happy about.

Bar stiffness is easily obtainable in both materials, IMHO, as I´m able to obatin good martensite formation at quench.

Problem is which one will offer me better fatigue results.

Other companies that are actually supplying VW use the 34Mn5 , with excelent behaviour in the fatigue test (they do more than double the specification) and 47 HRC.

But our company is a subdivision of a bigger company which its main bussiness is tube manufacturing, and they have the 28Mn4, but NOT the 34Mn5. So being able to use the 28Mn4 would be very good news

Also, would you recommend a crude cold bending without stress release in front of an exigent torsion/flexion fatigue test??. Wouldn´t it create anisotropism due to grain orientation that could lead to brittle fracture growth??


Thanks you all guys, you are being so helpful, as you see I´m a bit too demanding with all this questions and explanations, but I just wan´t to have it all figured out. It´s my first project so I better do it without screwing the whole thing up!!!

 

[Neubaten]

ha, ha!!! the "sketch" is all wrong!!

 

[TVP]

Neubaten,

There will not be a significant difference in the fatigue strength of 28Mn4 vs. 34Mn5 provided that they are quenched and tempered to similar microstructures. Obviously 34Mn5 will have greater hardenability, and if the composition is near the high side of the range, it will be capable of higher hardness/strength and therefore higher fatigue strength. However, this difference will be minimal, and the processing parameters (austenitizing T, time at T, quench T/concentration/flow, tempering T/time, etc.) will have a greater effect. Good luck.

 

[Neubaten]

Does anyone have any info or any web resource or book where I could find some insight about resistance tempering with shorter tempering time??


I´ve just been talking with a guy who runs a heat treatment company, and he told me that this rapid resistance temper thing could be a bit unstable, as slight variables on t/T throughout production could lead to very different properties.

Still, I´m quite interested into shorten the temper process by any means.

 

[Tmoose]

I have a broken OEM tubular anti-roll bar from a buddy's 120 kmiles Porsche Boxster over in the corner of my cubicle in Massachusetts.
The break started from a heavily rusted area on the exterior. It looks to me like the section of the anti-roll bar that was most rusted resided in the bushing mounted on the chassis.

 

[swall]

You might consider higher temperature, shorter duration furnace tempering. Pyromaitre in Canada makes this equipment. At my previous employer, we put in a similar furnace to temper induction hardened fixed outers for CV joints. We were able to pass the required the cycle life, but our OEM customer shot it down.

 

[CoryPad]

swall,

Good advice, although TVP beat you to it by 4 days...

 

[swall]

Yep, he sure did--missed it completely!