Citroën Standards "Mechanical Manufacturing": steel treatments and hardness (March 1935)

In this document

Page 34 — Steel treatments

This standard from the Société Anonyme A. Citroën, dated March 1935, belongs to the "Mechanical Manufacturing — Machining & Assembly" series. It presents a table of "TECHNICAL INFORMATION ACCORDING TO TREATMENTS" listing about twenty grades of steel used by the company p. 34.

Each type of steel is identified by a current symbol (e.g., C1, C6, H1, H7, K5…) and, for some, an old symbol in Roman numerals p. 34. The columns detail, for each heat treatment — annealing (A), quench-tempering at 700°, 625°, 525°, 375° and 200° (N, B, C, F, P), case-hardening (G+T2), cyaniding (Y) — the temperature and the quenching method (water, oil, air) as well as the breaking load in kg/mm² p. 34.

Some examples noted in the table:

An observation specifies that "the hardness above the line is that of the surface, below that under the layer" p. 34.

Page 35 — Hardness measurement and conversion

This second sheet, also dated March 1935, offers a correspondence table between several hardness scales: resistance R (kg/mm²), Rockwell B (1/16 ball, 100 kg), Rockwell C (diamond, 60/100/150 kg), Brinell (5% ball at 750 kg and 10% at 1000/3000 kg) and Shore p. 35.

The values range from R = 260 kg/mm² (i.e., Rockwell C 88, Brinell 764, Shore 104) down to R = 34 kg/mm² (Brinell 99) p. 35. A final note specifies that these are "Average values" p. 35.

Context

In the mid-1930s, Citroën standardized its industrial processes through an abundant collection of internal "standards", intended to harmonize machining practices between workshops and suppliers. The Rockwell and Brinell hardness scales, developed respectively in the 1920s and at the beginning of the 20th century, were then commonly used in the automotive industry to control the quality of steel heat treatments.

This March 1935 document precedes Paul Magès' entry into Citroën by ten years: hired in August 1925 as a self-taught worker, he only joined the design office at the end of the 1930s, before drawing the first hydropneumatic suspension plans on 17 February 1944. This period corresponds to the years of the Traction Avant, whose design required a fine mastery of steels for transmission and suspension components — a field in which Magès would distinguish himself much later.

Archivist's perspective

This technical document, dating from more than a decade before Magès' major inventions, testifies to the documentary collection he kept or inherited as a Citroën engineer — having joined the company in 1925, he remained there for 49 years, until his retirement in September 1974 (Photographs of Paul Magès' retirement (1974)) — rather than personal work dated 1935. It illustrates the metrological and normative rigor of the Citroën house well before Magès' arrival at the design office.

Its interest lies in the preservation of pre-war manufacturing standards, to be compared with other technical pieces from the same collection such as the Time/speed conversion tables and hourly speeds in 3rd gear or the Technical notebook no. 1 on belt transmissions, which fall under the same culture of standardized engineering at Citroën.