Aero Engineering (Newnes) has a table from Hadfields Ltd. It is again not useful in that it places a hyphen under C, Si, Mn and Ni. It has S and P at .05. I am curious that Ex Brat quotes ‘Sulphur = not less than 0.05%, Phosphorus = not less than 0.05%’. Aero Eng notes that S and P above .06 causes problems. For S, the steel becomes brittle, for example. As discussed above, I expect that the hyphens just mean that it is not specifically controlled. I expect that the S & P values are actually maximums. I haven’t pasted a picture because for some reason I can’t upload at present.
Looking at the Vampire Mk.30 SRM (AAP.851), I see that there are potentially some S.2 parts: G00653 Eye Bolt is listed as S.11 or S.2 bar. No help of course for the XRF unless you can figure which option they actually used. Some De Havilland parts do have the material spec marked on them.
I suspect that unless you cut up the part you are testing, XRF would not tell you if a part had been case-hardened if you only tested the outside surface; this goes back to having the drawing which would specify if the part is carburised or nitrided, to know the truth about composition.
There are a number of factors behind the decision to select a forging over a simply machined part, or one assembled or welded. To consider the initial thought in design: a forged part may be the lightest solution or it might be the most compact solution where the design has to be compact – if a part has too much bulk then required clearance to adjacent parts might be sacrificed – think in particular of moving parts that require clearance to surrounding fixed structure. After that initial thought the next one is whether the manufacturer has the capability to make forgings or can get the forgings supplied. The manufacturer may chose to invest in what is for them a new process. Later on, a manufacture develops a standard practice amongst their models where a certain undercarriage part, say, becomes standard in general design – the process has been developed, and the investment in the forge and process development needs to be paid off. Each new aircraft model that comes along, the manufacture uses the IP they have developed, and uses the same basic idea and manufacturing process. This may hold until the part actually has additional requirements placed on it or a great new process comes along that has benefits worth investing in, or even that the manufacturer discovers that after the part has been in service 15 years that it actually has shortcomings.
In terms of modern replacements for forgings, there is a compromise that modern airliner manufacturers use. These are forged blocks. A basic forged block may be fully machined to make an elevator hinge fitting, say. The benefit is that a hinge fitting is basically carrying a pin joint with a spherical bearing, so that the load directions on the lug are limited and moments aren’t transferred (different though at the actuator). The benefit then in the forged block is good directional properties. Although machining does cut the aligned properties the overall process is cost effective.
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