From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of spring plate materials, including hardness tests, chemical composition, and the effects of decarburisation.
| Identifier | ExFiles\Box 154a\4\ scan0057 | |
| Date | 9th December 1937 guessed | |
| -2- M.1,486/8 - Upper surface of small American spring plate, after removal of .002" depth of material. Note patchy distribution of ferrite at this level. These should be compared with :- M.1,486/9 - Upper surface of Firth's spring No.2 after removal of all decarburisation to reveal normal structure beneath. Hardness Tests. (120 Kg/ 2mm. ball). Tests made in centre of transverse sections. Woodhead's Spring No.24 - 380 Brinell. Firths Spring No.1 - 420 Brinell. Firths Spring No.2 - 423 Brinell. American Spring - 415 Brinell. Analysis of American Spring. Carbon - 0.55% Manganese - 0.83% Silicon - 2.02% These figures fall just within our Specification 402 - Silico-manganese steel. The carbon and silicon are near our maximum limits and the manganese is near the minimum limit. Effects of Decarburisation. Even slight decarburisation reduces the fatigue endurance limit very considerably, as we know from experience not only with road spring and valve spring materials, but with steels generally. In this connection it will be of interest to quote from a paper by Dr. Hankin and Dr. Becker (National Physical Laboratory) Journal of the Iron and Steel Institute :- "It has been shown that the fatigue resistance of spring steels is profoundly influenced by the conditions of the surface of the material. Thus in ordinary laminated springs used in automobile suspension systems, the steel is in the form of unmachined heat-treated rolled plates, and for this condition of the material it has been found that the endurance limit may be appreciably less than one half of that of the same material when machined and polished". These investigators quote the endurance fatigue limit for heat-treated but unmachined silico-manganese steel specimens with a decarburised surface as ± 20 tons/sq.in., as against ± 47 tons/sq.in. for specimens ground and polished after quenching. They also applied various experimental heat treatments intended to prevent the loss of resistance to fatigue due to surface action during heat treatment, having found that normal furnace treatment of ground and polished specimens reduced the endurance limit to ± 27 tons/sq.in. | ||