Tuftriding/Nitro-carburised Crankshaft/Conrods.

[Bullman]

1- Could someone explain to me the process of each Tuftriding,Nitro-carburised,Heat treated,Nitrided.

2-What would be the benefits if I had my x-flow crankshaft and conrods engineered with one of the above processes.

3-Does anyone know an engineering firm who could do the above preferably near to Yorkshire.

Thanks in anticipation Bullman.

 

Bullman

[oilyhands]

I'm no metallurgist but my understanding is that tuftriding and nitriding are heat/immersion processes which chemically harden the outside skin of the material, tuftriding can be applied to low grade steels and cast iron, nitriding or nitro-carburising can only be applied to special nitriding steel like EN40B. Induction hardening is done by heating the material using a controlled current and is very accurate WRT depth of treatment and temperature achieved so gives consistent results.

 

Tuftriding and nitriding help improve the fatigue life of components as well as providing a toughened skin. Shot-peening is a common treatment for rods, the surface or skin of the rod is micro compressed which lends a sort of negative stress to the outside of the rod this improves strength and fatigue life.

 

It's worth tuftriding the crank and maybe shot-peening or tuftriding the rods, following turftriding the journals will need to be polished since they 'grow' a little with treatment.

 

Oily

[R2D2]

As "Oily Hands" says both processes involve the diffusion of "interstitial alloy elements" into the surface of a steel component.

 

There are two basic Tuftriding process one known as Austenitic Nitro-carburising and the other known as Ferritic Nitro-carburising. It the Ferritic process which is of interest because it is relatively low temperature and avoids the distortion that is normally associated with more conventional case hardening. Austenitic Tuftriding takes place at about 850degC and Ferritic Tuftriding at about 550/575degC.

 

Tuftriding gives a very shallow case depth but can be applied to most steels and cast irons. The main benefit is to produce surface compressive stresses which inhibit the initiation of fatigue cracks. (In general terms 90% of a components fatigue life is during the crack initiation phase. The increased hardness of the surface is a function of the development of the compressive stresses.

 

These compressive stresses are all important because any crack initiation and growth process needs tensile stresses to operate and surface compression reduces the magnitude of the tensile stress developed in a material for a given applied force.

 

Tuftriding produces a "white layer" on the surface of a component that should not be removed before use. It is this white layer that gives the benefit to component life.

 

Nitriding is probably the best process but as has been stated does need steels with a high Aluminium and Molybdenum content. EN40B is the best known nitriding steel for crankshaft applications. it is the formation of aluminium nitrides that provides the surface compression and high levels of hardness that are associated with this process. Nitriding general egineering steels should be avoided because the production of iron nitrides on the surface of a material can reduce fatigue life and cause failures to occur more easily than in untreated components.

 

The nitriding process is quite complex and relies on the use of a fully heat treated component. The stages of hear treatment would be:

 

Harden and temper a rough machined EN40B crank ( I would leave a 2-3 mm machining allowance) to approximatley "T" condition. (800MPa) This would invlove oil quenching from 850degC and tempering at 600/650degC.

 

This heat treatment would cause quite significant distortion, which is why it needs quite a large machining allowance.

 

(If you were turning a "billet" crank you could buy material in a "T" condition.)

 

The next stage of the process would be to carry out final turning with about a 0.5mm allowance and then "stabalise". Stabalising involves heating to about 575degC for 2-3 hours depending on the size of the component. The objective of stabalising is to allow for all residual stresses to be removed before grinding.

 

The next stage is to grind the component to almost finished size. nitriding then takes place at about 550degC. The nitriding tempertaure is always slightly lower than the stabalising temperature and this should pervent distortion due to any further stress relaxation.

 

There is a slight amount of growth during nitriding and as in Tuftriding a "white layer" is produced. The big difference is that the white layer produced by nitriding must be removed before the component is used. This is usually removed by grinding and so the small amount growth is often a sufficient final grinding allowance.

 

Nitriding is much better than tufrtiding and gives much higher compressive surface stresses and deeper cases but does have a much higher cost due to the special materials that are needed.

 

I would agree that shot peening rods is a good idea and better than heat treatment but it needs to be done by a specialist. Again the concept of shot peening is to introduce surface compressive stresses but research has shown that if the shot is not applied perpendicular to the surface in a very controlled manner tensile stresses can be produced. This would significantly shorten component life.

 

There are two other problems.

 

Even if the correct fixtures are used it is possible to bend a component if there is too much work introduced onto one side and it is necessary to build up the srface treament evenly, a small amount at a time.

 

It is necessary to use the correct grade of shot and have the correct screening procedure to remove broken particles. I have seen shot that has been sold commercially as suitable for shot peening, which is much too angular and would damage surfaces and intrduce pitting. These effects would reduce fatigue life.

ou really need to be sure that the sub-contractor uses an Austenitic stainless steel shot and has the correct screening system.

 

I would recommend Metal Improvements Ltd. in Newbury Tel 01635 279600 as they have the correct technology and experience.

 

Good Luck

 

 

[Roger King]

I'd just like to add two things to the comprehensive replies.

 

We have been tufftriding standard crankshafts for many years with excellent results (longer bearing life, longer crankshaft life).

 

The finish of the journals as tufftrided is too rough to run without polishing, BUT the surface treatment is very shallow and it is very easy to polish it away - seen it done many times!