Replacing Forged Components With Hipped Castings

[Riley Imp]

May I ask for some advice. I am researching the merits of replacing Riley pre-war forged steel rockers with investment cast components, hipped to enhance their strength. I understand this technique has been used with success on vintage JAP engines. The Riley rocker is a simple beam design with a central bearing for the rocker shaft, a hardened pad for following the valve stem and a slotted end to provide a means of using a screw to clamp the pushrod rocker pin after adjustment. This slot provides a stress concentration which often fails, leaving the pin to part company. Racing use with higher revs. also sees a failure of the beam at its thinnest point, next to the pad. I would like an input from someone who may have some knowledge of value to us. How good is hipping, how effective is it in dealing with stress concentrations. Better or worse than forged ?,
All help gratefully received, Chris

[imull]

anything that increases the cros sectional area of a component will add strength to it.

Not sure about forging vs cating of components except that most performance con rods are forged I think.

[Rubinho]

Forging should be stronger than casting, all other things being equal. Cast items often have a degree of porosity (unless they're die cast under pressure) which weakens the metal. Forging counts as hot work, strengthening the metal by aligning the grains along the lines of stress.

An increase in c.s.a. will strengthen a part.

HTH

[BugEyed]

Some basic information on "hipping" may help. Dad is trying to see if there are any people here who have experience of using this process to replace costly forged parts with the lower cost hipped alternatives.

Anyone with experience that they are willing to share?

Duncan

PS Also know as Riley Imp's son.

[Riley Imp]

I should have perhaps explained. Hipping refers to Hot Isostatic Pressing, a heat treatment process for aircraft engine cast components to improve strength. It improves intergranular strength to take a cast component to the equivalent of a fine grain forging. As one guy put it " it makes all the molecules in the structure hold hands. "
What I am not sure of, is the downsides. Is it for example, more susceptible in fatigue to stress concentrations caused by sharp edges etc. It would be reassuring to know that it is no worse than a conventional forging.
Chris.

[KC]

Even die casting components have porosity. It will never be fully eliminated as gasses will always propigate from the base material in castings. HIPping sounds a lot like stress relieving, standard procedure when hardening hot work tool steels. During any kind of metal working, stress builds up in the material. Extreme hot or cold can be used to relieve this stress and force the material to adopt its new shape without warping or cracking.

There is a process used when casting aluminum wheels that does something similar to this. During the dwell time in the mold as the metal is changing from liquidus to solid state hydraulic cylinders are driven into the mold to improve the molecular density, especially in areas of high porosity and poor mold fill.

If you take the material and heat it and then work it in a press to compress it and increase its density, then you are hot forging it. Thats essentially how tackle blocks and crane hooks are manufactured.

[BugEyed]

Thank you for your experiences and knowledge. The devil is in the detail - how do these components fare in high stress areas such as holes and reduced cross-sectional area?

Any other metalurgists out there?

Duncan :confused:

[autosportfan]

As many of the repliess have mentioned casting does leave slight porosity in the metal. The amount of porosity is a factor of the size, shape of the part and the casting system, ie, position of runners and feeds. There are two types of porosity, the first is gas porosity, caused by metal-mould reactions or gas evolution. The second is shrinkage porosity, caused by the contraction of the metal as it cools. HIPping applies a pressure at high temperature, which acts to close the shrinkage porosity and reduce the size of gas porosity (this will not dissappear as this type of porosity contains gas and as the size gets smaller the pressure increases until it stablises with the applied pressure of the HIPping process). Generally, HIPping does NOT increase strength (unless the original casting was v. bad), but it increases fatigue life by reducing the number of stress concentrations (the pores).
This process should not be used to makes a bad design good, only to improve a good design.
Two ideas you should consider, change the design to lower the in service stress and secondly consider a stronger material.

[BugEyed]

Thank you for the helpfull information. The idea is to use a material with better properties and to improve the design, but the cost of a forging is something we'd like to avoid if we can get similar results with a HIPped casting. I guess that as ever the devil is in the detail. Do you have any idea where we can get data on the fatigue life of HIPped cast components versus forged components?

Thanks in advance.

Duncan

[Revracing]

How many rockers are you looking to replace? Investment casting is very expensive for small runs, I would agree with autosportfan, if there are no restrictions on doing so, the prefered route would be to re-design and machine from billet post heat treat if necessary. As has been previously said casting have inherant flaws in their makeup, but what hasn't been mentioned is that forgings are far from the ideal, when highly stressed the material of a forging has a greater tandancy to deflect (change shape) than a machined or cast component

[Riley Imp]

Thank you autosportfan. We have been offered cast materials close to the original forged materials with similar levels of UTS and proof strength. % Elongation was lower, but as investment cast, fatigue life was significantly lower. Original materials were En 40 for standard components, but KE965 had been used for racing models. Even those tend to fail after sustained high revolutions. All failures I have examined bear the signs of fatigue. HIPping was promised to restore fatigue strength to that of a forged component and we are currently looking at materials that might provide a more consistent and higher level of UTS. As mentioned earlier the weakest point in the original design lies in a slot cut to allow a pinch force to be provided on the rocker pin. The idea behind this was to adjust tappet clearance and then clamp with a screw which pulls the two pieces either side of the slot together. Post-war, Riley recognised the fragility of the design and on a bigger engine resorted to a lock nut on the pin doing away with the slot in the process. They also had a larger rocker box cavity to fit this in. Having studied the space envelope of the earlier 9HP engine we are dealing with, I think we can incorporate a modification to the rocker design which will allow a locking nut. We can also improve the cross-section of the I beam of the rocker body. Thanks again for your advice.
Chris.

[Riley Imp]

Sorry Revracing, I was punching the key-board whilst you were posting yours, Bugeyed as well !
Tooling costs are low, because we have a club member who can help. I would hope to sell around 50 engine sets a year through the club spares company. This means 400 rockers per year. I base this on what we sell by way of valve sets, rocker pins, shafts etc. Members have been pressing me to come up with a solution for this component.
Chris.