What alloy was used in early steam turbines? (1900 - 1930)

Question

Steam turbines use externally created steam to drive the blades. There's no internal combustion. They've been used in marine propulsion since at least 1894 on the Turbinia. After this, Parsons-type steam turbines became more and more popular on navies.

I want to know what the blade alloy was, back then. A decent blade alloy is crucial, or the turbine will melt or creep or corrode too quickly.

This is important to trace the similar technology of turbojets developed in WW2. I want to see if the alloys of those early jet engines are similar to the marine steam turbines.

I did google around for this, but was disappointed. Wikipedia has an entire article on steam turbines, with very little on blade alloy. It had a few small paragraphs describing Ni-Al-Ti alloys (without mentioning ratios), and also protective coatings, without specifying any time-frame. Usually that means it's a modern context, and the citation dates seem to support that.

Another thing I found was this Wikisource Article. The only time it mentioned blade alloy was a 1888 turbine from Sweden, "a paddle wheel made of the strongest steel". Hardly a scientific description. What was the strongest steel in 1888? What % of carbon was it? There's only a 1% difference of carbon between wrought iron and steel. Besides, this doesn't appear to be the parsons-type turbine that became so popular on navies about a decade later. And there is no other mention of blade alloy in the article.

Answer 1

Parsons Turbines

The early Parsons turbine blades were actually made from brass, or from pure copper for higher steam temperatures. In the 1994 Parsons Memorial Lecture, J R Bolter observed:

On the early turbines the blades were of brass, first stamped and then rolled to shape, with pure copper blades for higher steam temperatures to avoid embrittlement. As peripheral speeds rose and stresses increased, steel blades were introduced, and in 1925 ductile stainless iron was introduced as standard. This material developed into the 12 per cent chromium steels which are used for almost all steam turbine blading today.

Sir Charles Parsons himself, in the 1911 Rede lecture said:

"The blades were cut off to length from brass, hard rolled and drawn to the required section, and inserted into a groove with distance pieces between and caulked up tightly."

• p 11

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Parsons steam turbines were fitted, for example, in many of the British K-class submarines during the First World War.

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Construction and constraints of brass blades

I found some further details about the construction of the turbine blades in John William Sothern's The Marine Steam Turbine; a practical description of the Parsons and Curtis marine steam turbines as presently constructed, fitted, and run.

The blading for Parsons' turbine is of brass, and is manufactured by various firms; it is usually delivered in lengths of from 5 to 6 ft. The turbine proper being formed of blades of various lengths and spacing, which are termed "expansions," the blades are cut to lengths in a machine which shears them off and at the same time stamps a double or treble groove on the end.

• p 82

Sothern also makes the important point that the brass blades would expand more than the steel housing, which would reduce the clearance of the blade-tip:

The blade tip clearance (cold) for the above varies from 49/1000 in. at the 1st expansion to 50/1000 in. at the 4th or last expansion; when heated up, however, the actual blade tip clearance is only about two-thirds of the forgoing at the 1st expansion, and rather less at the last expansion, the brass blades expanding more than the steel rotor drum or the cast-iron casing. The cruising dummy clearance cold is only about 15/1000 in., but generally, when heated up, this increases to about 25/1000 in., or even more. The steam, after passing through the first cruising turbine, enters the second or M.P. cruising turbine, if one is fitted, then the H.P. ahead and L.P. ahead turbines, exhausting finally to the condenser. If only one cruising turbine is fitted to each set, the steam exhausts from it direct to the ahead H.P. turbine, which is the arrangement in the " Indomitable "-" Inflexible " class.

• p 47

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Blade alloy

As for the specific brass alloy in use, James Ambrose Moyer, in his 1908 book The Steam Turbine; A practical and theoretical treatise for engineers and designers commented:

It is stated that the usual alloy used in England for blades of Parsons turbines is 63 Cu 37 Zn; but any zinc alloy is quite unsuitable for superheated steam or for high velocities.

• p 114 (my emphasis)

He also stated that Monnot metal was beginning to be used in steam turbines by that date:

Recently a compound metal known as Monnot or "duplex" metal has been developed . It consists of a steel core covered with a thin copper sheathing chemically welded to the steel in such a perfect manner that the blades may be drawn cold from the original ingot into the required finished section without in any way affecting the bond between the copper and the steel.

• p 112

Such blades were then being used in Westinghouse turbines.

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de Laval's experiments

In the Rede lecture cited above, Sir Charles Parsons mentioned the 1888 Swedish turbine you mention, but it seems that de Laval was driving a paddle-wheel made from steel with a jet of high-pressure steam, not an actual turbine-blade:

"In the year 1888 Dr de Laval of Stockholm undertook the problem with a considerable measure of success. He caused the steam to issue from a trumpet-shaped jet, so that the energy of expansion might be utilized in giving velocity to the steam. Recent experiments have shown that in such jets about 80 per cent, of the whole of the available energy in the steam is converted into kinetic energy of velocity in a straight line, the velocity attained into a vacuum being about 4,000 feet per second. Dr de Laval caused the steam to impinge on a paddle wheel made of the strongest steel, which revolved at the highest speed consistent with safety ..."

• op. cit., p6 (my emphasis)