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).
Article on the application and testing of Calorized refinery pipe still tubes.
| Identifier | ExFiles\Box 150\1\ scan0097 | |
| Date | 28th March 1935 | |
| 44 THE OIL AND GAS JOURNAL March 28, 1935 Application of Calorized Refinery Pipe Still Tubes By C. L. CLARK¹, ROGER STEWART BROWN² and A.{Mr Adams} E.{Mr Elliott - Chief Engineer} WHITE³ While a steel must always possess sufficient strength or load-carrying ability to withstand the applied stress without undergoing more than a definite, fixed mation, there are also many cases which a good degree of oxidation or corrosion resistance is necessary. Results from creep tests and from commercial practice have shown that certain of the low-alloyed, pearlitic type steels possess sufficient creep strength for continuous service at temperatures of 1,000 to 1,200° F.{Mr Friese}, and also for short-time over stressing at considerably higher temperatures. These steels usually do not possess a suitable resistance to oxidation and, in general, they are badly attacked by the sulphur compounds in sour crudes. In order to obtain the necessary degree of resistance against sulphide corrosion and severe oxidation, it has been common practice to resort to the relatively high-alloyed steels. This procedure has two disadvantages. First, steels of this type are usually expensive and the savings resulting from their use may be largely offset by the increased initial costs; and second, certain of them possess certain inherent detrimental properties which limit their field of usefulness. It appears, therefore, that the field of application of certain of the low-alloyed, pearlitic steels could be widely extended, especially in connection with the refining of sour crudes, if their surface were to be protected by some material which was highly resistant to both sulphide corrosion and oxidation. Aluminum is known to fulfill this condition, not only as a virgin metal, but also when it is alloyed with steel, provided, of course, that a sufficient amount of aluminum is present in the iron-aluminum alloy. The formation of this iron-aluminum compound on the surface of steel is known as Calorizing. It should be emphasized that Calorizing is not a mere plating or covering of steel with aluminum as a definite solution of the aluminum in the iron is obtained. soon as this happened, the protecting action of the layer was destroyed. New Method A new method of Calorizing was recently developed⁴ which eliminates this difficulty as it produces a coating of considerable thickness which has sufficient ductility to permit a relatively large deformation without cracking and suitable shock resistance to withstand the thermal stresses and the action of the tube cleaner. This method consists of two processes. In the first, the part to be Calorized are heated in a closed retort with the usual Calorizing mixture. They are Calorized in the usual manner and an iron-aluminum alloy is formed on the surface which contains 50 to 75 per cent aluminum over a depth of 0.005 to 0.010 inch. This step is represented by the shaded rectangular section of Figure 1. This layer, however, is brittle, because of its high aluminum content, and there is a distinct line of demarcation between the aluminuminized surface and the underlying steel, along which there is a tendency for the surface layer to spall off if the article is subjected to deformation. The Calorized parts are then placed in closed retorts and heat treated. This causes a penetration of the iron-aluminum alloy into the material, as represented by the triangular section of Figure 1. In this way the high aluminum concentration of the thin surface alloy is reduced to a point (about 30 per cent A1) where the alloy becomes sufficiently ductile to permit the required deformation, and the line of demarcation is entirely obliterated, thus leaving no plane of weakness. The depth of penetration, and thus the thickness of this layer, varies somewhat, depending upon such factors as the chemical composition and grain size of the steel being used, but it averages approximately .040 inch. The two photomicrographs of Figure 2 show typical micro-structures of this layer at a magnification of 100 diameters. The one designated as (a) is a plain carbon steel of the 1015 type, while the other is a low-alloy steel of the chromium-molybdenum-silicon type, known commercially as Timken DM.{D. Munro} The extreme outer surface, varying in thickness from 0.0076 to 0.009 inch, is somewhat porous, but it is the underlying material which imparts the oxidation and corrosion resistance. Properties of Materials While the present Calorized layer possesses a sufficient degree of ductility for most applications, it will, of course, crack under excessive deformation. For the most satisfactory service it is, therefore, necessary to select a base material which possesses sufficient creep strength or load-carrying ability to withstand the applied stresses. In other words, the base material supplies the strength and the Calorized layer, the oxidation and corrosion resistance. In order to determine more fully the properties of this coating when applied to various low-alloyed steels, a rather expensive series of investigations were undertaken. Standard Creep Tests In order to determine the actual load-carrying ability of these Calorized steels at elevated temperatures, creep tests were undertaken on the standard units of the University of Michigan, which are capable of detecting elongation changes of 2.8 millionths of an inch. The single-step method of loading was employed with at least four different stresses being used for each steel at each temperature. All tests were continued for at least 1,000 hours. The specimens were first machined to size, except for the threaded portions, and were then Calorized. The threads were cut after the Calorizing operation. The specimens had a gauge length of 2 inches and a diameter of 0.505 inch. The results obtained from certain of these tests are given in Table 1. The steels considered are mild carbon steel of the 1015 type, two carbon-molybdenum steels which contain 0.50 and 1 per cent molybdenum, respectively, and Timken DM{D. Munro}, which is a chromium-molybdenum-silicon alloy. For comparative purposes results are also included at 1,200° F.{Mr Friese} for a steel of the 4-6 Cr.{Mr Cra???ster / Mr Chichester} + Mo type. The temperatures considered ranged from 1,100 to 1,400° F.{Mr Friese} as it is believed tube fractures often occur at these more elevated temperatures, due to coke deposition causing overheating. Although values are included for three different creep rates, it is believed that for this type of service greater emphasis is generally given to the creep rate of 1 per cent per 10,000 hours. If the steels are compared on this basis, a rather wide variation exists in the stresses required at each temperature, depending upon the alloying content. At 1,200° F.{Mr Friese}, steel DM{D. Munro} --- Image Captions & Text: Fig. 1—Diagrammatic sketch of the two steps in the calorizing operation Fig. 2—Typical micro-structures of the calorized coating enlarged to 100 diameters. Upper, plain carbon steel type 1015. Lower, chromium - molybdenum - silicon alloy known as Timken DM.{D. Munro} Fig. 3—Comparative load-carrying ability of calorized and uncalorized steels at 1400° F.{Mr Friese} and 2000 lbs. per sq. in. load. K.{Mr Kilner} S. Killed Mild Steel C-Mo. Carbon-Molybdenum (.50 Moly.) D.M. Timken D.M. Steel Cr.{Mr Cra???ster / Mr Chichester}-Mo. 4-6 per cent Chromium plus .50 Moly. CALORIZED SPECIMENS K.S. FRACTURED 99.5 HR. ELONGATION 41.2% C-Mo. FRACTURED 480 HR. ELONGATION 15.0% D.M. NOT FRACTURED 600 HR. ELONGATION 6.4% I Mo. NOT FRACTURED 600 HR. ELONGATION 2.24% UNCALORIZED SPECIMENS K.S. †FRACTURED 59 HR. ELONGATION 79.4% C-Mo. †FRACTURED 110 HR. ELONGATION 43.7% D.M. †FRACTURED 230 HR. ELONGATION 21.7% Cr{Mr Cra???ster / Mr Chichester}-Mo. †FRACTURED 487 HR. ELONGATION 40.1% --- Footnotes: ¹Research engineer, department of engineering research, University of Michigan. ²Superintendent, the Calorizing Co. ³Director, department of engineering research, University of Michigan. ⁴U. S. Patent 1,988,217. †Partly caused by reduction of section by scaling. | ||