US2995441A

US2995441A - Production and treatment of pearlitic malleable cast iron

Info

Publication number: US2995441A
Application number: US732768A
Authority: US
Inventors: Rubel Werner
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-04-29
Filing date: 1958-04-28
Publication date: 1961-08-08
Anticipated expiration: 1978-08-08

Description

United States Patent 2,995,441 PRODUCTION AND TREATMENT OF PEARLITIC MALLEABLE CAST IRON Werner Riilrel, Velbert, Rhineland, Germany, assignor to Walter Overholf, Velbert, Rhineland, Germany, a corporation of Germany No Drawing. Filed Apr. 28, 1958, Ser. No. 732,768 Claims priority, application Germany Apr. 29, 1957 10 Claims. (Cl. 75-123) 2.80-3.l% carbon 0.40-1.20% silicon 0.30-1.20% manganese, preferably 0.30-0.60% manga nese 0.l2-0.35% sulphur Up to 0.10% phosphorous 0.01-0.5% titanium 0.00-0.5% Chromium Balance iron C-haracten'm'ng features of this novel composition are the titanium content thereof and also the relative proportion of the manganese, sulphur, titanium and chromium.

Cast iron of the aforementioned composition is adapted for conversion to malleable iron having good structural state and excellent strength properties in the annealed and also in the hardened or ennobled hardened and drawn state. Any of the heating treatments heretofore employed in the production of malleable iron can be employed in malleabilizing the cast iron of the invention. A feature of the malleable iron of the invention is that it can have an extremely fine lamellar pearlitic structure or an extremely fine globular pearlitic structure.

The malleable iron can in general be formed from the cast iron in greatly reduced annealing times. This factor can be of great significance since the required annealing time for malleabilizing is commonly an important or even controlling factor in respect to the economics of production of the material. A further advantage of the novel malleable iron is that it can be fusion welded. Hence, the malleable iron is suited for use in fusion welded structures.

The cast iron of the invention suitable for conversion to malleable iron, is produced by charging a cupola having a non-acid lining of basicity from neutral to semibasic, with ferrous material and slagging material suitable to form a semi-basic slag. The charge within the cupola is maintained at a temperature suflieient to fuse the ferrous material and form a slag of the slagging material, and the slag is maintained semi-basic. Ferrous melt formed within the cupola is withdrawn and cast to form a cast metal of the aforementioned composition which is suitable for malleabilizing according to the invention. The composition of the ferrous melt, and therefore the cast iron, is controlled by controlling the composition of the cupola lining and/or the compositions of the materials charged to the cupola.

The titanium content of the cast iron can be obtained by the use of titaniferrous pig iron or other titanium bearing material.

Preferably, the cupola employed is a semi-basic or neutral cold-blast cupola, and corresponding semi-basic slag regulation is employed.

By the expression semi-basic or neutral cold-blast cupola, is meant acupola furance which, deviating from the normal acid lining, has a lining which is semi-basic or neutral, i.e., inthe case of the semi-basic lining, there is an addition of A1 0 or AMO -containing ores, namely 8 to 25% and preferably 18 to 25% A1 0 based on the lining material. in small quantities, MgO and TiO In the case of the neutral lining, graphite in more or less coarse pieces is included in the Otherwise, it consists essentially of silica. In this case also the SiO, generally contains small amounts of TiO,.

By semi-basic slagging is meant the preparation of a slag, the final analysis of which is approximately the following:

8-30%, preferably 10-20% A1 0, 40-60%, preferably 45-55% SiO; 20-35%, preferably 20-30% CaO As remaining components, the following will as a rule be present:

About- 1-5% MnO 1-4% FeO Up to 3% TiO, Up to 2% M30 1-2.5% P 0 Very fine pearlitic malleable iron can be produced from the cast iron by annealing treatments heretofore known for the production of malleable iron from cast iron but which treatments are in the case of the invention considerably shortened but are of a time suflicient to form the desired pearlitic structure. While being malleabilized, the ledeburite structure of the thoroughly white cast iron (having no graphite in this state) disintegrates into fine lamellar pearlite and temper carbon. When annealing is continued, the lamellar pearlite is by and by converted into globular pearlite.

This is achieved without any prolonged annealing near the transformation temperature and without recourse to the process of temperature alternation above and below the transformation temperature, but may be efiected at the malleabilizing temperature in short time.

By the expression fine lamellar pearlitic structure is meant a structure having cementite and ferrite in form of very thin and short lamellas; by the expression fine globular pearlitic structure is meant a structure in which the lamellas of the cementite are spheroidized.

The presence in the metal composition of the invention of the alloy elements, particularly the titanium, chromium and sulphur, has the eifect that it favors the formation of ledeburite, but at the same time the ledeburite formed from the metal compositions decomposes extremely easily upon the annealing so that the advantage is obtained that annealing times are relatively short.

The metallic composition of the invention is particularly well suited to treatment designed to provide in the castings, surfaces free of ferrite and which are completely pearlitic. To obtain surfaces of this nature, according to the invention, the castings are treated while present in a neutral environment and preferably in a salt bath. This manner of treatment is to be considered in contradistinction to the customary methods of malleabilizing in an oxidizing atmosphere in which a temper ore is used as a source of oxygen and this oxygen burns the carbon present at the skin zone of the casting, and also in contradistinction to the nowcommonly used treatment of gas malleabilizing.

The salt bath should if possible have an evaporation point above 950 C. Suitable salt baths are those containing sodium carbonate, chlorides, particularly barium Patented Aug. 8, 1961. t

The balance consists of SiO; and,

3 chloride and possibly sodium cyanide and potassium cyanide. There has been found to be suitable a bath additionally containing molybdenum sulfide, particularly molybdenum disulfide, as well as of sodium sulfite and barium sulfite, singly or in any combination. In this way malleable cast iron having a ferrite-free skin zone can be obtained.

The treatment temperature in the salt bath is between 760 and 1100 C. and preferably between about 950 and 1000 C. One can proceed in various manners. It is possible to effect the entire treatment in the salt bath or in a plurality of salt baths, the first of which for instance is set at a temperature between 950 and 1000 C. and another bath at 760 to 950' C. It is furthermore possible to provide only a short time of stay in the salt bath, for instance 30 to 80 minutes, and to thereupon place the treated pieces in a furnace. v This furnace should have a controllable atmosphere, exclusion of air or inert gas. The stay in the salt baths or in the furnace is'dependent on the cross-section of the pieces to be treated. However, in the case of exclusive salt-bath treatment, the duration will be less than 10 hours, and depending on the height of the temperature, even considerably less. Also upon after-treatment in a furnace, of for instance 760 to 850 C., the periods of time are considerably reduced and are at most between about 8 and 15, hours.

The tensile strength of the malleable cast iron treated in this manner is, without heat treatment, between about 50 and 65 kg./mm. with a yield point of about 32 to 48 kg./mm. and an elongation of about 3 to 20% (L=3d). By heat treatment, yield points of between 70 and .120 kg./mm.' at tensile strengths of 80 to 160 kgJmmP'can be obtained.

The cast iron can also be excellently carburized and/or surface-hardened or through-hardened. In particular when decarburizing of the surface is to be avoided, it is advisable to effect not only themalleabilizing but also the heating to hardening temperature in the salt bath. One can proceed in the manner that hardening is effected at a suitable temperature directly from the bath in which the malleabilizing was efiected.

Referring again to the composition of the cast iron and malleable iron of the invention, it will be noted that the stoichiometric sulphur-manganese ratio common to malleable castings is substantially higher than is true for the metallic material of the invention, and that the material of the invention is of relatively high sulphur content and at the same time, of low manganese content. While in the case of. previously known malleable cast iron compositions, the rule of approximation applies that the manganese content should be at least twice or at least three times the sulphur content. In the case of the material of the invention, lower ratios are employed. Thus according to the invention, the manganese content is preferably less than about 1.7 times the sulphur content.

In the case of compositions of the invention of low sulphur contents, i.e., between about 0.12 and 0.2%, a chromium content of 0.02 to 0.5% and preferablyseveral tenths of a percent is provided, and where the sulphur content is higher than 0.2, the chromium content is advisedly kept very low, for example from to 0.09%.

The relationship of titanium, manganese and sulphur in the composition can be expressed by the following formulae:

Ti (percent max.)=0.2% Mn Mn (percent max.)=l.7% S

The phosphorus content of the composition of the invention is preferably maintained below the upper limit of 0.1% and it is highly desirable if this content is within the limits of 0.03 to about 0.07.

The production of cast iron and the production of malleable iron according to the invention will now be described with reference to working examples. These examples are ofiered in order to illustrate in the inven- 4 tion and are not intended to set forth the limits of the invention.

Example I I The operation is carried out in a cupola, the lining of which, in addition to Si0,, also contains about 15% A1 0; 1% TiO; 4% C 2% M30 1.5% FeO 1.5% MnO The mix also contains the recycle material of the titaniferous grade produced here. Basic-Bessemer steel scrap was also added. In this case, one refrained from using FeMn or FeSi, which may definitely enter into consideration in other cases. The mix accordingly contained the following percentages by weight:

Percent Pig ir n 14 Recycle material 42 Scrap 44 In addition to this, there is included as slag-forming addition per 350 kg. of metallic charge 20 kg. CaCO; 8 kg. A1303, and 15 kg. $10,

In this connection, there was obtained a malleable crude cast iron of the following composition:

2.9% carbon 0.63% silicon 0.31% manganese 0.24% sulphur 0.06% phosphorus 0.08% titanium 0.05% chromium The final slag analysis belonging to it was as follows:

56% sio, 9.8% Ago, 23.64% 0110 4.31% no 0.84% "no, 0.7% MnO 1.35% MgO 1.3% 150,

From this crude cast iron there were produced machine-part castings, the pieces being made with a wall thickness of 5 to 45 mm., and these parts were malleabilized at temperatures of 960 C. in a slightly oxidizing atmosphere for 60 hours, plus heating-up time and cooling-down time. Comparable parts of normal malleable cast iron required an annealing time of about hours. The parts to be hardened, after the malleabilizing process, were heated to 850 C., held at this temperature for 3 to 5 minutes and then quenched in cold water. In order to increase the toughness, the parts were tempered to 380 0., time of tempering 3 to 5 minutes. The technological properties prior to the heating and quenching were as follows:

Yield point kg./mm.- 32-36 Tensile strength kg./mm..... 52-58 Elongation percent.. 6-8

with a cross section of 12 mm.

The technological properties after the tempering were as follows:

Yield point kg./mm.' 80-90 Strength "kg/mm?" 120-135 Elongation pcrcent 1-3 Another part of this crude casting was malleabilized in a salt bath, namely in a bath having the following composition:

The malleabilizing temperature was 950' C. and its length 1 hour, followed by annealing for 12 hours at 850' C. At this time the yield point was 48-52 lag/mm, the tensile strength 66-75 kg./mm. and the elongation 4 to 5%.

The matrix in this case was of very finely globularpearlitic structure with extremely finely divided temper carbon. No special heat treatment was efiected here since the strength properties were suflicient for the purpose of use even without special heat treatment. Otherwise, it could have been followed directly by a heat treatment which would have given comparably higher values for the yield point and tensile strength with the same treatment than in the case of the above described cast iron malleabilized in the furnace.

Example II In this example, the melt was formed in the presence of a neutral lining.

In a cupola of approximately identical dimensions to that used in Example I, there was provided in the hearth and nozzle zones a lining of about 60% graphite Balance substantially SiO,

A mix was selected with titaniferous pigiron as in Example I.

To this there were added 52% recycle material and 38% chromiferous steel scrap For each 350 kg. of this mix there were added kg. CaCO; 6 kg. A1 0, 15 kg. SiO,

6 The malleabilizing was effected by annealing at about 960 C. as in the first example. The technological properties of this cast iron were, in malleabil-ized state:

Yield point", kg./mm. 36-38 Tensile strength "kg/mm-.. 51-56 Elongation percent.. 7-11 with a test bar of 12 mm. o, referred to a measurement length of the test bar of 3 xdiameter.

I claim: a

1. Cast iron consisting essentially of the following constituents in the amounts indicated based on the total of said constituents:

2.8-3.1% carbon 0.4-1.2% silcon 0.3-1.2% manganese 0.l2-0.35% sulphur Up to 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance iron further characterized by the fact that the manganese content is less than 1.7 times the sulfur content, and the titanium content is less than 0.2 times the manganese content.

2. Cast iron according to claim 1, with the proviso that for a sulphur content less than about 0.2%, the chromium content is at least 0.2% and for a sulphur content greater than about 0.2% the chromium content is less than about 0.09%.

3. Cast iron according to claim 1, the manganese content being less than about 1.7 times the sulphur content, the titanium content being less than about 0.2 times the manganese content and the phosphorus content being in the ranage of about 0.03 to about 0.07%.

4. The method of producing cast iron suitable for conversion to pearlitic malleable iron by annealing for a relatively short period of time which comprises charging to a cupola having a non-acid lining of basicity from neutral to semi-basic, ferrous metal, and inorganic slagging material suitable to form a slag, maintaining the cupola at a temperature sufiicient to fuse the ferrous material and form a slag of the slagging material, and collecting a ferrous melt in the cupola, the compositions of the ferrous material, cupola lining and slagging material bei: g such that at the temperature maintained in the cupola the melt consists essentially of the following constituents in the amounts indicated based on the total of said constituents:

2.8-3.1% carbon 0.4-1.2% silicon 0.3-1.2% manganese 0.124135% sulphur Up to 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance iron annealing said cast iron for a relatively short annealing time, whereby fine pearlitic malleable iron is formed.

5. The method of claim 4, wherein the slag comprises 8 to 30% alumina, 40 to silica, and 20 to 35% calcium oxide.

6. The method of claim 4, wherein the said sl' neutral and comprises graphite silica and titaniur oxide.

7. The method of claim 4 wherein the slag additi ly contains 1-5% MnO, l-4% FeO, up to 3% Ti( to 2% MgO and 1-2.5% phosphorus pentoxide.

8. The method of producing a very fine globular pearlitic malleable iron by employing a relatively short holding time at the hardening temperature which comprises heating at the hardening temperature a previously malleabilized iron essentially of the following constituents in the amounts indicated based on the total of said constituents:

2.8-3.196 carbon (IA-1.2% silicon 03-12% manganese 0.12-0.35% sulphur Up to 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance iron for a time sufficient to form a very fine globular pearlitic I indicated based on the total of said constituents:

2.8-3.1% carbon 0.4-1.2% silicon (LB-1.2% manganese 0.l2-0.35% sulphur Up to 0.10% phosphorus 0.01-0.5% titanium 0.00-0.5% chromium Balance iron in a salt bath for about 30 to 80 minutes at a malleabilizing temperature and thereafter heating in a furnace in the presence of a neutral environment atv a malle abilizing temperature.

10. The methodot producing pearlitic malleable iron suitable for use in fusion welded structures which comprises heating a cast iron consisting essentially of the following constituents in the amounts indicatedbased on the total of said constituents:

2.84.196 carbon 0.4-1.296 silicon 03-12% manganese (LIZ-0.35% sulphur Up to 0.10% phosphorus (ml-0.5% titanium 0.000.5% chromium Balance iron Fletcher et a1 June 15, 1926 Schwartz Aug. 30, 1927 FOREIGN PATENTS France Mar. 22, 1956

Claims

1. CAST IRON CONSISTING ESSENTIALLY OF THE FOLLOWING CONSTITUTENTS IN THE AMOUNTS INDICATED BASED ON THE TOTAL OF SAID CONSTITUTES:

2.8-3.1% CARBON 0.4-1.2% SILCON 0.3-1.2% MANGANESE 0.12-0.35% SULPHUR UP TO 0.10% PHOSPHORUS 0.01-0.5% TITANIUM 0.00-0.5% CHROMIUM BALANCE IRON