US2319538A - Heat treatment of copper-chromium alloy steels - Google Patents

Description

2,319,53s NT OFFICE I naA'r'raaa-rrramoF'corrEa-cnnomn ALLOY STEELS ,Wiiliam Pollard Digby, London, England, assignor of one-half to Everard Tuxford Digby, London,

England No Drawing. Original appncauon June 28, 1937,

Serial No. 150,881. Divided and this application 1 August 6, 1940, Serial No. 351,651

, Claims. This application is adivision of my application Serial No. 150,881, filed June 28,1937, which issued February 4, 1941, as U. S. Patent No.

2,230,531. Y 1 l i This invention relates to improvements in the treatment of alloy steels containing copper and chromium. a

Various alloy steels are known containing copper in proportions of 3 1% or 2% and it has also been proposed to make alloy steels containing chromium and over 5% of copper. such steels containing over 5% of copper have not come into commercial use, as no satisfactory method has been published for the fabrication of a satisfactory range of useful articles.

My copending application now United; States Letters Patent 2,230,531 has been addressed to processes of manufacture of alloys and preparation of articles from such alloy steels. It is the object of the present invention to provide such alloys characterized by a. novel structureand to provide worked articles of such alloys having novel physical properties.

Alloys containing over 5% of copper and over of chromium may be prepared in known manner, and they exhibit stain-resisting properties. If however, it is attempted to roll or forge such alloys at the usual temperatures employed for similar stain-resisting chromium alloys containinglittle or no copper, especially nickel chromium steels, it is found that the higher copper alloys are burnt, become'cracked and exhibit the phenomenon of hot-shortness.

Such undesirable results are preservedif it is attempted to forge the alloys at temperatures of 1400 to 1500" C., and even as low as 1200 C.

I have now discovered that these difliculties can be overcome if certain precautions are observed in mechanical working, though special methods of annealing and heat-treatment (for hardening) are also desirable, and these special methods of treating worked articles differ pro- However,

foundly from those applicable to the well known not exceed 1000 0., although some result can be obtained at 1025? C. with extreme care and risk of wastage. Broadly.v speaking, the best range of initial working temperatures is from 800 to 900 C. i p

Unlike alloy'steels of similar composition, the

copper-chromium steels can be mechanically worked as low as 700 C. or even" 650 C. without danger of cold-shortness. However, for economic reasons it is obviously desirable to start at an initial temperature of 800 and preferably higher, otherwise too frequent re-heating would be necessary.

The invention further comprises certain methods of annealing and heat treatment as hereafter described in detail. Whereas nickel-:chromium steels can either be annealed by quenching or hardened by slow cooling, the properties so obtained are not usually reversible, i. e. a soft material cannot be converted into a hard one after the treatment. According to my invention, annealing is effected at a temperature of say 750-970 C., and is usually followed by hardening heat-treatment consisting of quenching or air-cooling and the hard and soft properties so obtainable are reversible.

It has been found that when an ingot of the above alloy steel is obtained by slow cooling of molten metal, the ingot possesses a 'dendritic structure, and it has further been found that 'forwards between the same rolls since reversal of direction would destroy the parallelism of the long crystals. I

In a casting the dendrites are initially radial. Rolling in one direction gradually causes them to. lie as long very fine fibres, ultimately at right angles to their original direction, that is finally they will be parallel with the direction of rolling. If rolling is performed in a direction at right angles, this should be at an intermediate stage.

Similarly in hammer cogging the working must be always in one direction along the ingot.

If the molten metal is allowed to cool rapidly the crystalline structure is polyhedral. In this case the material can still be mechanically worked between the stated temperature limits, but the final product will not possess a fibrous structure and will not be suitable for certain purposes for which the fibrous material can be used. v

In a preferred form of the presentinvention, a billet of a copper-chromium, alloy steel is allowed to cool slowly to obtain a'dendritic crys talline structure of long coarse crystals,whereupon the billet is passed to a soaking pit for reheating and is mechanically treated, for instance, by hammer cogging the ingot from its centre to its ends, whilst the ingot is at a temperature from 800 to 900 C. A particularly suitable temperature for the cogging treatment has been found to be 850 C.

After this treatment it has been found that the bar forged from the ingot or other mechanicalelement so forged, can be re-heated over a wide range; that is to say, from 900 C. to 1050 C. to allow of further mechanical treatment as by forging into a crank shaft or other mechanical article, or again, for rolling into strips or plate.

The forged or rolled article may then be subjected to an annealing treatment, preferably from 750 to 970 C. varying with the percentages of the varying metals, .for instance 910 C. has

been found suitable for 15% chromium in an 8 to 10% copper chrome steel.

The lower temperatures-are desirable for ease in pickling, while the higher temperatures are preferred if maximum softness is desired.

The finished article may be further treated by reheating and either quenching in water or air cooling, the quenching temperature varying with the copper chromium content.

For a content of 15% chromium and 10% copper the temperature for maximum hardness is substantially 500 (2.

As-illustrating the difference in properties obtainedloy quenching from different temperatures, I may mention that quenching from around the AC point (about 970 C.) gives a very hard alloy of high tensile strength, whilst from around the AC point (between 500 and 600) gives steel of lesser hardness, slightly lower tensile strength but greater toughness.

The mechanical and heat treatments as set forth above are particularly applicable for chromium-copper-a'lloy' steels in which the chromium content is analogous to that of the known nickel chromium steels; the corrosion-resistance increases progressively from 10% to 15% chromium or even 17-19% chromium, for commercial reasons, the upper limit of chromium content is about 20 to 25% though higher percentages may be used.

The minimum copper content is about 5-6% and a preferred lower limit is 7'-8%. The preferred range is 8-10%. The copper may be as high as 20%, or even 40-50% but that is an obvious economic upper limit. Large increase in copper raises ductility at the expense of corrosion-resistance. For deep-drawing alloys 15-25% of copper may be present.

The balance of the alloy is principally and almost entirely iron, though small quantities may be present of metals used as additions to steel, and in particular of metals which .alloy with copper. The content of any such addition is preferably less than 1-2%. Nickel may be present as an accidental impurity (less than'1%) but it has little or no useful effect, and the nickel content should be less than2%.

The carbon content should be low; by sacrificing part of the stain-resistance, a forgeable alloy may be made with up to 0.3% C. but the C content is preferably less than 0.12%.

Copper chromium alloy steels subjected to a treatment as above, and having a chromium content of 15% or less will be found to be air hardening whilst those greater than 15% will not be air hardened. It is preferable to vary the heat treatment with the percentage of chromium.

Chromium copper alloy steels treated in the fibres extending substantially parallel to one manner of the present invention will be found to have high ductility, a high tensile strength and considerable resistance to corrosion forming "stainless" steels having similar properties to nickel chromium steel, and of a similar carbon content; that is to say, usually from 0.05 to 0.1% carbon.

The properties of the alloys can be varied within wide limits and the optima in certain directions, for particular properties (not necessarily combined in the same sample) are as follows:

Firstly, the ratio of the yield point to the maximum stress-may be between 70% and Secondly, the izod impact values of the alloy maybe from 50 to ft. lbs. (forgings).

Thirdly, a percentage elongation of over 30% (2-inch test-piece) on a steel having a tensile strength of 30. English tons down to a steel having "an elongation of 22% having a tensile strength of .52 English tons.

Fourthly, the alloys do not work-harden under carbon and the balance substantially all iron, and

characterized in having a dendritic structure with the fibres oriented substantially parallel to one another and to a surface of the article and in being capable of successive heat-treatments for hardening and annealing the same.

2. An easily; machinable stain-resisting article of analloy containing copper 8 to- 10 percent, chromium 10 to 19 percent, carbon less than 0.12 percent and the balance substantially all iron, and being characterized in having a dendritic structure with the fibres oriented substantially parallel to one another and to a surface of the article, and being further characterized in being capable of successive heat-treatments for hardening and an ealing the same.

3. An easily machinable stain-resisting copperchromium-steel alloy containing 5 to 20 percent of copper and 10 to 19 percent of chromium, the balance being essentially all iron so that iron constitutes the major ingredient of the alloy, said alloy being characterized in possessing a fine dendritic sructure with fibres extending substantially parallel to one another.

4. An easily machinable stain-resisting copper-chromium-steel alloy containing copper 5 to 20 percent, chromium 10 to 19 percent, less than 0.3 percent carbon, and the balance essentially all iron, and characterized in possessing an "essentially all iron, and having an Izod impact value of 50 to 120 foot-pounds combined with a tensile strength of 30 to 52 English tons, and characterized in having the properties of easy machining ability and of not work-hardening, and of possessing a fine dendritic structure with another.

WILLIAM POLLARD DIGBY.