US1712251A - Manufacture of rolled-steel members - Google Patents

Description

' Patented May 7, 1829'. V

NITED STATES ilian PATENT, OFFICE;

NABAINA DAS GHOPRA AND FREDERICK JOHN BULLEIN', OF WESTMINSTER, LONDON,

ENGLAND.

MANUFACTURE OF ROLLED-STEEL MEMBERS.

No Drawing. Application filed December 10, 1926, Serial No. 154,027, and in Great Britain January This invention relates to the manufacture of rolled steel members, and for the sake of convenience will be described by reference to the manufacture of steel rails as typifying rolled steel members. It is to be understood,

however, that the invention is not limited to the manufacture of steel rails but is applicable to the manufacture of any form of rolled steel member.

Tn the ordinary manufacture of steel rails there occurs a zonic heterogeneity of structure which may be said to be characterized by the presence of three zones, namely, a central zone of pearlitic nature, an outer zone of soft dcca-rburized steel, and an intermediate zone of high-carbon steel. This zoning of structure is generally accompanied bya state of internal instability, and has been found to be mainly responsible for the faults commonly experienced in rails, of

corrugation and excessive wear and tear in use.

It is known to be possible to remove from steel defects of the kind referred to above,

,2? by annealing, i. e. by a treatment comprisproducing softening of the steel; it does not eliminate from the structure of the steel the aforesaid zonic heterogeneity, and, further, it leads to the formation of scale,'with an accompanying loss of steel by weight.

According to the present, invention, a. process of manufacture of rolled steel members is characterized in this that the completely formed member is submitted to a heat-treatment which comprises heating the 40 member from a temperature below the-lowest change-point of'the metal to a temperature above (e. 50 C. above) the highest change-point, cooling graduallyvto a temperature somewhat below (e. g; 50 C. below) the lowest change-point, and thereafter more rapidly cooling, for example, by allowing the member to cool spontaneously in the atmosphere. By highest change point and lowest change point we mean the highest and lowest critical pointson heating and cooling, respectively; In the claims these critical points have been designated, re-, spectively, as Ac and Ar.

The effect of the specialheat-treatment them. The process of this inventionmay be applied as a final heat-treatment of a rolled steel member which has been subjected-to the heat-treatment described in the specification of our British Letters Patent No. 271,580 of 1927, in the event of said heat-treatment failing to result in complete elimination or suficient reduction of the said defects.

Preferably, the high tenrlperature heat- :treatment of the membe is effected in the presence of an oxidation inhibitor, by which is meant a solidmatcrial which is non-fusible at the temperature employed in the heattreatment and the action of which, through chemical affinity of the material towards the deleterious furnace gases, is to protect the metal from oxidation or substantially to reduce the degree thereof. Examples of oxidation inhibitors are alkaline earth oxides, such as lime; infusible readily oxidizable materials in a condition to present to the gases a large surface relatively to the area of the exposed surface of the member under l'ieat-treatment, for example shavings or turnings of an oxidizable metal such as steel; and carbonaceous material such as an exhausted carburizing mixture, boiler ash or producer ash, containing an adequate proportion (for example at least 10%) of combustible carbon.

By an exhausted carburizing mixture is meant a carburizing mixture which has been used to such an extent as to render it unsuitable for further use in carburizing. A suitable carburizing mixture which, when exhausted, has been found to be utilizable according to the present invention consists of three parts of powdered beech charcoal andtwo parts each of powdered horn charcoal and powdered animal charcoal. The boiler ash or producer ash may he of the type which usually contains from 5 to 15 per cent of-carbon.

' The use in heat treatmentof' steel of a mixture of alkaline earth oxide and an appropriate carbonaceous material containing free carbon such as an exhausted carburizing reagent, forms the subject of our British Letters Patent 271,606 of 1927, and is claimed herein only in conjunction with the specific heat treatment which characterizes the the nature of the member. Our general cxperience to indicate that such degree maybe relatively small. For example, we have attained the advantages aforesaid by heating to approximately 50 C.-above the highest ch ange-point and gradually cooling to C below the lowest change-point. We have found that the degree to which the 'member should be allowed gradually to cool below the lowwt change-point is generally not less than 50 C.

We have frequently found in our work upon steel members thatthe process; of this invention givjes specially good results in the case of steel containing a small proportion of a metallic element, such as nickel or cobalt, the presence of which tends to depress the change-points of the steel.

According to a further feature of the invention, therefore, the steel member subjected to the aforesaid special heat-treatment is composed of steel containing a small proportion of a metallic changeoint depressant, such, for-example, as nic el or cobalt;

The proportion necessary of such element is preferably quite small, for example 1% or less. We have usually found 0.5% to be sufiicient. In this connection it will be unment characteristic of this invention is specially effective in the case of chrome steels; and according to a further feature of the invention the steel member subjected to heattreatment as'. above is composed of chrome steel. We have found that with this particular'special metal the advantageous effectsof the invention are frequently very marked.

. The chrome steel from which the member is formed may with further advantage be a chrome steel containing a change-point dcpressant such as nickel or cobalt.

Example I An ingot was cast from a steel assaying as follows Per cent. Carbon 0. 62

Silicon 0. 177 Sulphur O. 0355 Phosphorus -i 0. 04 Manganese 0. 86 Nickel n D y 0. 54 The tapping temperature was 1520 0.

The teaming temperature with one inch nozzle) was 1450 C.

The top of the ingot mould was covered during setting with an oxidation inhibitor consisting of an exhausted carburizing mixture.

On stripping, the ingot was coated with a further quantity of the aforesaid oxidation inhibitor. The stripping temperature was 960 C.

The ingot was transferred to the soaking pit, the floor of which was covered to a depth of about three inches with a mixture of lime and boiler ash carrying 15.6 per cent of combustible carbon, the proportion of limeto ash being approximately 40:60, and the'sides of which were loosely packed around the ingot, with steel turnings. The space beneath the lids of the pit between the top of the ingot and the roof of the fur nace was also filled in with loosely disposed .steel turnings suspended from the lids of the pit.

The temperature of the ingot was raised in the soaking pit to1190 G. whereupon the ingot was transferred at the temperature stated to the cogging mill. The temperature during cogging was 1090 C.

The billet, after cogging, was then subjected' without further heat-treatment to rolling operations to form the rail and the finishing temperature of the rail was 995 C.

The completely formed rail was then allowed to cool on thehot-banks to a temperature of approximately 500 C., after which it was heated again in a re-heating furnace to approximately 815 C. and thereafter allowed gradually to cool in this furnace to about 540 C. The rail was then removed from the furnace and allowed to cool spontaneously in the air. The reheating and slow cooling treatment in the furnace was effected in the presence of an oxidation inhibitor similar to that employed in the soaking pit, the furnace being packed with the inhibitor in the same way.

The structure of the rail showed itself to be in a homogeneous pearlitic form and the rail exhibited no sign of a dccarburized zone at the surface.

Example I l This is an example of' the manufacture of a rail from a chrome steel, the analysis The operations of pouring, teaming,'cogfit ging and rolling were carried out as in the Example I, the temperature of the ingot during/ cogging being 1090 0. and the finishing temperature after the last roll being 995 C. The rail after the last roll was allowed, as in Example I, to cool down on the hot banks to about 500 C. after which it was heated again through the changepoints to approximately 815 C. and then cooled slowly in the reheatingfurnace to about 540 C. At this temperature the rail was removed from the furnace and allowed to cool spontaneously in the air to normal temperature. The reheating furnace contained an oxidation inhibitor as in the case of the furnace of Example' l. As in the case of Example T the rail showed'no signs of zoning of structure.

l t was found that without sacrifice of tensile strength both the elongation and the reduction in area of test piece were very materially increased by the special treatment characteristic of this invention as compared with the effects of ordinary treatment involving a mere hot-bank cooling of the finished rail. The following are the strength figures for the rail llllaximum t e n s i l e strength Q. 50. 36 tons per sq. inch Elongation on 3" l5. 0%

Reduction in area of test piece 31. 6%

The corresponding figures for an ordinar i ily manufactured rail of exactly the same Maximum t e n s i l e' strength 50.48 tons per sq. inch Elongation on 3" Reduction in area of test piece In addition to the improvement indicated above in the figures for elongation and reduction in area, the rails produced according tothe above examplesbehaved better in a very markeddegree in the drop tests. Indeed, the difierence in behaviour in the drop tests was even more marked than the improvement shown in the tensile strength tests. In the case of the chromesteel rails manufactured by the ordinary process, nearly every rail broke under the drop test at the second blow, whereas of the rails manufactured according to'the present invention the majority stood more than three blows at the drop test without fracture.

Reverting again to the oxidation inhibitors the presence of which as aforesaid is preferable during the heat-treatment which characterizes this invention, the inhibitor may comprise sheets or plates of an oxidizable metal such as iron or steel disposed around the walls of the furnace or beneath the roof thereof or in the gas openings. Alternatively, the steel member to be protected may be enveloped during the heattreatment in a cage built up of sheets or plates or bars of iron or steel or other readily oxidizable metal.

Again, if an alkaline earth oxide such as lime be employed as the oxidation inhibitor it may be employed in powder form or in aqueous suspension or solution. ner of applying the oxide may be varied according to circumstances and convenience. If the member under heat-treatment is the initial hot ingot, or is in the billet stage or, again, is proceeding through the rolling mills, the oxide may be sprayed or dropped upon the member. On the other hand. if the member is undergoing heat-trcatmentin a soaking pit, reheating furnace or other furnace, the member may be placed in a bath of powdered lime or like oxide or a paste of the oxide may be coated upon the member.

In some cases it may be found preferable either as an alternative to the aforesaid forms and modes of application of oxidation The man inhibitor or in conjunction therewith to line the soaking pit or other furnace with doloinite or furnace slag.

Lastly, with regard to the oxidation inhibitors, any two or more of the aforesaid forms may be employed conjointly.

It is to be understood that the present invention is applicable to the treatment of a defective rail which has been produced by the ordinary methods of manufacture for the purpose of eliminating the defects.

We claim 1; A process of treatment of rolled steel members which comprises heating the fullyformed member from a temperature below the lowest change-point of the steel to a temperature slightly above the highest Ac change-point by direct contact with furnace gases which have been rendered substantially innocuous towards the steel as regards their oxidizing and decarburizing capacity by treatment with an alkaline earth oxide and a substantially non-carburizing carboncarrying material as herein defined, cooling gradually to a temperature slightly below the lowest Ar change-point in the presence of material'capable of inhibiting oxidation of the steel, and thereafter allowing the member tofinish cooling in an uncontrolled manner. v I

2. A process of treatment of rolled steel members according to claim 1, wherein the relative amounts of the oxide and carboncarrying material are such that the quantity of oxide is equivalent to from 10% to 50% by weight of the combined quantities of oxide and carbon-carrying material and the,

carbon content of the carbon-carrying mate-. rial issueh that the quantity of free carbon presentis equivalent to from 4% to 20% by weight of the aforesaid combined quantities. 3. A process of treatmentof rolled steel members according to claim 1, wherein the heating operation is effected in the presence of a false increase of area of oxidizable material provided by a mass of infusible readily oxidizable material, substantially as and for the purpose set forth.

4. A process of treatment of rolled steel members, which comprises heating the fully formed member by direct contact with hot furnace gases through the Ac changepoints of the steel to a temperature of about C. above the highest change-point, in a furnace containing a sufiiciency of lime and substantially non-carburizing carboncarrying material as herein-defined to render and maintain the furnace gases substantially lnnoeuous towards the steel as to their-oxidizing and decarburizing capacity, ceasing the heating operation and allowing the member to cool gradually to a temperature of' about 50 C. below the lowest Ar change-point in the presence of material capable of inhibiting oxidation of the steel, and thereafter allowing the member to finishcooling in an uncontrolled manner.

5. A process of treatment of rolled steel members, which comprises heating the fullyformed member by direct contact with hot furnace gases through the Ac change-points of the steel to: a temperature of about 50 C.

above the highest Ac change-point, in a furnace containing a mass of infusible readily oxidizable material in a form to provide in the furnace a relatively large false increase of oxidizable surface area exposed to the'furnace gases and a sufliciency of lime and substantially non-carburizing carbon-carrying material as herein defined -to render and maintain the furnace gases substantially innocuous towards the steel as to their oxidizing and decarburizingcapaeity, ceasing the'heating operation and allowing the member to cool' gradually to a temperature of about 50 C. below the lownee of material capable of inhibiting oxidation of the steelfand thereafter allowing the member to finish cooling in an uncontrolled manest Ar change-point in the pi'ese ner.

members, which comprises heating the full yformed member by direct contact with hot furnace gases through the Ac change-points ofthe steel to a temperatureabout 50 C. above the highest Ac change point, in a furnace containing a sufficiency of' lime and substantially non-carburizing carbon-carrying material as herein defined to render and maintain the furnace gases substantially innocuous'towards the steel as to their oxidizing and decarburizin capacity, ceasing the heating operation anif allowing the memher to cool gradually in the furnace and in the presence of the lime and carbon-carrying material to a temperature about 50 C. below the lowest Ar change-point, and thereafter allowing the member to finish cooling in an uncontrolled manner, the relative amounts present ofthe lime and earbOn-carrying material being such that the quantity of lime is equivalent to from 10% to 50% by Weight of the combined quantities of lime and carbon-carrying material and the carbon content of the carbon-carrying material is such that the quantity of free carbon present is equivalent to from 4% to 20% by weight of the aforesaid combined quantities.