US2184926A - Metal blasting material and method of producing the same - Google Patents

Description

Dec. 26, 1939. G. H. KANN 2,184,926

METAL BLASTING MATERIAL AND METHOD OF PRODUCING THE SAME Filed July 28, 1938 2 Sheets-Sheet 1 3nnentor ,Gusfal/e H. Kan/7..

(Ittornegs.

G. H. KANN Dec. 26, 1939.

METAL BLASTING MATERIAL AND METHOD OF PRODUCING THE SAME Filed July 28, 1958 2 Sheets-Sheet 2 Zhwentor Gusfave h. Kan/7. Mr

(Ittomegs memes Dec. 26, 1939 METAL BIASTm-G MATERIAL AND METHOD OF PRODUCING THE SAME Gustave H. Kann, Pittsburgh,- l'a., assignor to The Globe Steel Abrasive Company, Mansfield, Ohio, a corporation of Ohio Application July 28,1988. Serial N- 2213.! 8 Claims. (Cl. 51-280) This invention is directed to metal blasting material and method of producing the same. The material in question is particularly applicable for use in machines generally known in the industry as metal blasting machines or "sand blasting" machines. The material is most applicable for use in machines wherein the metal blasting material is applied by mechanical 10 means rather than by air pressure to the surface to be treated. However, the material is not limited to use in the mechanical machines and may be used in the machines which operate by means of an air blast.

Itisoldinthearttousesandasamedium for cleaning metal surfaces such as the surface of castings or the surfaces of metals which have been heat treated and are coated with a scale. It is also old in the art to use metal shot and Q0 crushed metal particles or metal grit. There are certain difllculties which are experienced with the materials which have been used up to the present time and this invention seeks to overcome some of those difllculties. Shot made of cast iron analyses by various processes have been extensively used in the industry. Such shot are usuallychilled rapidly and have a white iron structure. Crushed material made of chilled cast iron shot, or other cast iron products, have been used for metal cleaning both in the air-blast equipment and in the mechanically operated equipment. Likewise, crushed steel and fine particles of steel made by several processes have been used for metal cleaning. For example, it is known that the trimmings made from sharpening nails have been 'used for this purpose. In the case of these various materials there are objectionable features. and some of these will be mentioned hereinafter.

40 In the case of chilled iron shot, which are prepared by the usual commercial methods, the shot which are hard and brittle tend to crush up or explode when used in present practice, particularly when used in the mechanical metal blasting equipment. Breaking up and exploding of the shot causes the product to become too finely divided for satisfactory performance and therefore uses excessive amounts of themetal blasting material. The line material resulting from crushing of the shot must be removed from the system and new material added and this requires time and delays the work. Such hard material as chilled iron shot also increases the wear on the equipment used in this operation. It is therefore one of the objects of this invention to produce a much tougher shot, that is, one thatis not fractured in operation and it is of some importance to produce a shot which does not wear the equipment so rapidly. For this type of work.

shot ofsomewhat diiferent-degrees of hardness are indicated; thus, it is an object of this invention to be able to produce shot of controlled degrees of hardness but at the same time of sufg ilcient toughness that the shot does not fracture in service. Cost of the material is an important consideration and for that reason cast iron is one of the most suitable materials. The cast iron may be in m the form of rounded shot, produced by the commercial processes which are now in use or it may be in the form of crushed material or "gritf, which is made by such an operation as crushing the large particles of shot which are too large 15 for use in the present processes. Other forms of chilled iron may also be crushed and used.

In the accom g drawings: Figure l is a ew of the microstructure of shot at a magnification of 100- diameters.

Figure 21s a view of the microstructure at 1000 diameters. t

Figure 3 shows the microstructure of one form of applicant's shot at, 1000 diameters after heat treatment. a

Figure 4 is another illustration of the micro- 25 structure of another form of applicant's shot at 1000 diameters after heat treatment.

The chilled iron shot as now produced is quite hard and brittle and the microscopic examination of such cast iron shot shows that the structure is largely made up of a hard microconstituent martensiteand a still harder microconstituent, cementite, which is a compound of iron and carbon. Both of these microconstituents are a brittle and this is especially true of the carbides or the structure known as cementite. It is, therefore. an object of this invention to change this microstructure and thereby produce a shot which has a much greater degree of toughness but with the hardness controlled within certain i r desired ranges.

The microstructure of certain prior art chilled iron shot is shown in Figures 1 and 2 of the accompanying drawings. This lot of shot, which may be designated as Lot or Sample No. 2, has a chemical composition, as determined by analysis, of 3.37 per cent C, 0.40 per cent Mn, 1.62 per cent Si, 0.44 per cent P, 0.09 per cent Sand the remainder principally iron. This particular lot 50 of shot was of such selected sizes that the diameters range from 0.10'to 0.108 inch. The mlcrostructure represented in Figure l is at a magnification of diameters and shows more or less tempered martensite in the dark areas and 55 principally cementite or iron carbide in the light areas. The microstructure is more clearly shown inFigure 2 at a magnification of 1000 diameters in which the dark areas are marten'site with some retained austenite while the white areas are ceso mentite or iron carbide. These structures are known to be hard and brittle.

In the prior art, one patentee has su gested that iron or steel shot might be subjected to heat treatment processes to vary the hardness thereof ii. desired. However, not all iron or steel shot can be eifectively treated by the particular heat treatment processes utilized by me in the performance of this invention. Moreover, the term heat treatment processes is an extremely broad term and not all heat treatment processes will be effective to produce the results attained by me,

even though the chemical composition of the shot subjected to treatment would be suitable for use with my process. 4

It is one of the objects of this invention to treat certain. prior art shot or other shot. of proper chemical composition so as to transform the brittle structures characteristic thereof into tougher structures and, at the same time, to control the hardness of the shot within certain preferred ranges.

Another object of this invention is to provide a simple and effective method for treating shot of selected chemical composition so as to transform the brittle structures characteristic thereof into tougher structures and, at the same time, to control the hardness of the shot within certain preferred ranges.

It is another object of this invention to produce shot for. the purposes indicated which will be distinctly superior to prior art shot both from the standpoint of hardness and toughness, which will have superior effectiveness and longevity in use and which will at the same time decrease the wear upon the blasting apparatus utilized.

Other objects of this invention will appear as this description progresses and by reference to the appended claims.

While the term "shot" is used throughout this specification, it is to be understood that this term is intended to include irregularly shaped particles made by crushing or by crushing and grinding chilled iron or chilled iron particles which is generally referred to in the trade as fgrit.

My process contemplates the treatment-of shot of selected chemical composition by subjecting them to temperatures and other conditions of treatment which will effect partial malleableizing thereof or which will completely malleableize them. It also contemplates subjecting them to temperatures and conditions which will partly malleableize such shot and following such operations with a quenching operation.

The chemical composition of chilled iron shot as supplied to industry for metal cleaning or blasting varies considerably as is illustrated in Table I.

TABLE I.Chemical composition, of chilled iron shot 7 Chemical composition Lot No.

0 Mn Si P S 3. 31 44 2.02 44 .075 3. 37 40 1.62 44 09 3. 47 37 l. 42 51 095 3 47 35 l. 53 34 075 3 53 42 1. 83 39 085 31. 39 37 1.86 56 088 3 49 .45 1. 90 36 089 3. 29 34 l. 61 52 084 3. 46 44 1.99 .49 158 3. 44 40 2. 09 55 164 3. 48 47 1. 89 49 164 3. 33 39 2. 03 53 164 The above table shows the following ranges in chemical compositions: Carbon 3.29 to 3.53 per cent, manganese 0.34 to 0.47 per cent, silicon 1.42 to 2.09 per cent, phosphorus 0.343 to 0.559 per cent and sulfur 0.075 to 0.164 per cent. These ranges are only representative of a series of samples used in some researches and not limiting values. In fact, much wider ranges in chemical composition can be used in the production of chilled iron shot and will respond to heat treatment to improve the toughness and to control the hardness. The lower limit of carbon is fixed in part by melting practice and the stock commercially available. As the carbon content is decreased the temperature used in melting the charge must be increased. Because of this relation and the cost of melting stock, the low carbon limit is about 2.5 per cent for most purposes but carbon contents as low as 2.0 percent can be used. The upper carbon content is limited by the tendency of iron-carbon-silicon alloys to form graphite in the metal and thus weaken the material. Also, if the carbon content is too high graphite forms in the shot during solidification even when the shot is cooled rapidly as by a stream of water. Still other methods of producing chilled iron shot may be used in the production of shot suitable for use in carrying out my heat treating process. It is only essential that the shot be produced with a chilled iron structure and that the rate of cooling and the chemical composition be such that flake graphite is avoided or present in only a small amount.

These conditions limit the maximum carbon' content to about 4 per cent and if the silicon content is on the high side of the range then the carbon content must be held lower, as for example, at 3.0 to 3.5 per cent. The silicon content can be varied over a rather wide range, but its range is related to the carbon content. Silicon favors the formation of graphite during solidification and on cooling. It also favors the formation of graphite or temper carbon on reheating and too much silicon may cause undesirable results in heat treating the hard brittle shot to increase the toughness. In practice, it is generally necessary to use the low range of silicon with the high range of carbon and when the carbon is low the silicon content may be increased. While the average silicon content in the examples given in Table I is about 1.8 per cent, the maximum 2.09 per cent and the minimum 1.42 per cent, the possible range is much greater. For example,.in an iron of high-carbon content the silicon may be as low as to 1 per cent while with an iron of low-carbon content the silicon may be as high as 2% to 3 per cent.

The carbon and silicon are of primary importance in making white iron and iron that canbe malleableized. Therefore, it is essential to have the carbon and silicon contents so adjusted and controlled that essentially white iron is produced by the granulating and cooling process and that the iron will be capable of being malleableized. Higher carbon and silicon contents may be used with good results when the granulating and cooling is effected by pouring the molten metal from above into a ribbon-like stream of water under approximately pounds per square inch pressure which thereupon delivers it into a pool of water, than when a blast of steam is used for granulation.- Thus in carrying out my invention it is possible to use molten iron containing about 1.0 to about 3 per cent tents may vary over considerable ranges. While the manganese contents listed in Table I range from 0.34 to 0.47 per cent, the range may be considerably greater, especially higher. The manganese aids. in producing chilled or hard shot and slows up the graphitizing reaction. It also reacts with sulfur to form manganese sulfides which are less objectionable than iron sulfides. Thus, it is preferred to increase somewhat the manganese content when the sulfur is high in the iron. The preferred range of manganese is about 0.40 to 0.80 per cent but both higher and lower contents maybe used without materially interfering with my heat treating process.

The phosphorus'lowers slightly the melting temperature of the iron and increases its fluidity. On the other hand, too high a phosphorus content tends to make the chilled iron shot too brittle and may make the heat treated product too brittle for the best performance in certain types of service. Melting stock which is low in phosphorus may be more expensive. Thus it is deslrable to use a phosphorus content which meets as many of these desired requirements as pos-' sible and such ranges'asabout 0.30 to 0.60-per cent are preferred. Phosphorus contents well over 1 per cent'have been used in chilled iron shot with little or no loss in properties.

The sulfur contents listed in Table I range from 0.075 to 0.164 per cent. Sulfur aids in producing hard or chilled shot and the amount which may be present depends somewhat upon the manganese content, the higher the man- I ganese the higher the sulfur-may be permitted to be. The cost of melting stock may be higher if a low sulfur content is specified. The kind of coke or other fuel used in melting may influence the sulfur content. In general, a low sulfur content such as .05 to .08 per cent is satisfactory but higher sulfur contents such as 0.15 to 0.20 per cent do not seem to be particularly harmful in applying my heat treating process.

The data in Table I do not show the presence of such elements as chromium, nickel, copper and.

molybdenum. These elements are frequently present in the scrap used in the melting charge and as a resultare found in the shot. More detailed analyses than those reported in Table I have shown the presence of chromium, copper, nickel and molybdenum in the shot. The presence of small amounts of these elements does not seem to interfere with my heat treating process for toughening chilled iron shot or crushed material made from chilled iron.

. chromium is quite satisfactory and it is considered good practice in the production of iron shot for metal blasting to have'present small amounts of chromium such as up to 0.10 per cent. For shot which are to be retained in a relatively hard condition without having to resort to rapid cooling in the heat treating condition, chromium is desirmight be used for this purpose, chromium is less expensive.

Laboratory tests in shot from Lot 2 in Table I s Brinell hardness was 223 and specimens required able and may be used in amounts from about 0.05 to 0.50 per cent. Other carbide stabilizers but as a rule showed a Brinell hardness of about 680 and a resistance to crushing in static compression of 726 pounds. when tested under repeated impact blows it was foundthat an average of 6.7 blows with a hammer weighing 4.98 pounds and falling a height of 0.67 inch was required to fracture the shot of approximately 0.100 to 0.108 inch diameter. These data show that shot with microstructures such as'shown in Figures 1 and 2 possess high hardness and high compressive strength in static loading but that they are brittle when subjected to. impact as they would be in metal cleaning by mechanical or air blasting.

By applying my heat treatment-to samples of shot from Lot 2, I have been able to produce shot 20 of a wider range in hardness and especially to .produce shot with greatly improved resistance to fracturing when subjected to impact. Of the many heat treatments studied only a few will be given as illustrations.

- Example 1.A sample of shot from Lot 2 was heated for one hour at 1500" F. and then quenched in water. The Brinell hardness was 460. These heat treated shot required a load of 1000 pounds to fracture in static compression and withstood 80 over 200 hammer impacts without fracturing. The microstructure had been materially changed and was estimated to consist of the following in approximate percentages: troostite 72, cementite, 15, graphite 12 and ferrite 1. The microstructure is illustrated in Figure 3.

Example 2.The sample of shot from Lot 2 was heated one-fourth hour at 1500 F., one hour at 1400 F. and then water quenched. The

more than 200 hammer blows to fracture. The

microstructure was estimated as ferrite 66 per cent, graphite per cent, troostite 10 per cent and cementite 4 per cent. This heat treatment entirely removed the massive areas of cementite .and produced a product with good resistance to impact.

Example 3.In this heat, the shot were heated one hour at 1500 F., one hour at 1400 F. and then cooled in air. The Brinell hardness was 185 and specimens were not fractured by 200 hammer blows. The microstructure was estimated to consist of approxlrnatelyper cent ferrite, 18 per cent graphite, 5 per cent pearlite and 2 per cent cementlte.

Example 4.Another sample of Lot 2 shot was heated three hours at 1600 F., cooled at a rate of F. per hour to 1400" F. and then cooled with the furnace to about room temperature. Specimens from this test showed a Brinell hardness of and required an average of 88 hammer blows to cause rupture. By this treatment a'very soft shot was produced in which the microstructure consisted almost entirely of ferrite and graphite or temper carbon.

Example 5.-This sample of shot from Lot 2 was heated at 1350 F. for four hours and then quenched in water. Specimens showed a Brinell hardness of 146 and required an average of 141 hammer blows to cause fracturing. The microstructure showed approximately 78 per cent ferrite, 20 per cent graphite and 2 per cent cementite.

Example 6.This sample of shot from Lot 2 75 was heated at 1400 F. for one-half hour, cooled to 1350 F., and held one hour, being then water quenched. The resulting structure as shown in Figure 4 consisted of a ferritic matrix, a large amount of massive cementite and dark areas of graphite. Specimens showed a Brinell hardness of 228 and required an average of 109 hammer toughness of the material can be greatly improved.

Fundamentally, my heat treating process involves heating chilled iron shot'of selected composition, such as those previously mentioned, to

temperatures in the range of about 1300 F. to about 1600" F. for a suflicient period of time to break up the massive carbide or cementite and then cooling at a rate selected to produce the desired hardness. The time required decreases as the temperature is increased. For example, about four hours is required at 1350 F. to produce a shot which will be soft and tough on slow cooling while similar results can be obtained at 1600 F. in one-half hour andat 1500 F. in about one hour. Shorter heating times at these temperatures will give a product of somewhat higher hardness. In general, I further control the hardness of my heat treated shot by the rate of cooling from the high temperature. If

high hardness, such as 450 Brinell is desired,

then I find it desirable to cool the shot rapidly, as by quenching. For intermediate hardness I 0001 the shot more slowly as in an air blast, in still air or evenin the furnace. By these methods it is practical to produce metal blasting materials of a wide range in hardness but in all cases with improved resistance to fracturing when subjected to impact as in service. For example, all of the materials listed in Table I were heated one-half hour at 1500 F., cooled to 1400 F., held at that temperature for one hour and then quenched in water. All "of these lots except 7, 12

and 16 withstood 200 hammer blows without fracturing whereas before the heat treatment these same materials had fractured at about 2 to 12 hammer blows. There was a marked improvement in the impact resistance over the other lots as shown by the fact that Lot 7 increased frbm 3.? to 112, Lot 12 from 5.6 to 84 and Lot 16 from 1.8 to 154 hammer blows to cause fracturing.

In addition to laboratory tests, field and service tests have been made on shot heat treated according to my invention. In one test two thousand pounds of shot, representing a lot of one thousand pounds-of shot in the size range of about .047 to .056" and another lot of one thousand pounds in the range of about .068 to .078" diameter were heat treated according to my invention and used in a practical test.

These field tests showed conclusively that the heat treated shot produce much less wear on the mechanical equipment and that they last much longer in service than the usual commercial chilled iron shot.

, or its decomposition product martensite.

lic materials are used it is quite common in the trade to refer to the operation as sand blasting". As indicated earlier in this specification and illustrated in the examples, I control the proper- .ties and microstructures of the metal blasting material in part, by choosing the composition of the original product and in part by the heat treatment after granulation. It is desirable to select the composition of the material so that by the process of cooling the molten material I produce a product which is white or chilled iron; said product-having a microstructure consisting essentially of cementite or carbides and austenite This product should be essentially free from graphite and should show a white fracture. My heat treating process consists of several essential features. First, there is a heating and holding at temperature which is necessary to break up the massive carbides or to put them into solution or to do both.

At the low temperature I largely break up the carbides by decomposing them to form graphite or temper carbon. At the high temperatures I also effect partial decomposition of the carbides into graphite or temper carbon but I also eliminate the massive carbides to some extent by dissolving them in the austenite which forms at these high temperatures.

At the lowest temperatures which may be used in my process the rate of cooling has comparatively little efiect on the hardness. At the higher temperatures, however, where there is considerable carbon in solution the rate of cooling has a pronounced effect on the resulting hardness as shown in the examples. the composition represented by Lot 2 for one hour at 1500 F. and then quench in water, a

' Brinell hardness of 460 is obtained. The higher the'quenching temperature and the more rapid the rate of cooling, the higher the hardness will be. If a material is quenched from a lower temperature than 1500 F. the hardness will be lower. This is illustrated in Example No. 2, in which the material was quenched from 1400" F.

If a material of intermediate hardness is desired, I may, for example, heat to a relatively high temperature and then use a less rapid cooling rate as by cooling in air. Various modifications of these heat treatments are within the scope of this invention. For example, the material may be heated for one hour at 1500 F., cooled rapidly by water spray to a black heat and then permitted to cool more slowly. Such If I heat a material ofa heat treatment has certain advantages in that perature in order to produce a high hardness such as 400 to 500 Brinell and then reheat to a lower temperature in order to increase the toughness somewhat without greatly reducing the hardness. Still other modifications of the heat treating process might be employed and are considered within the scope of this invention. For example, the abrasive material may be quenched in a molten lead bath to rapidly cool the material to a temperature below a red heat and yet avoid a drastic quench. The quenching processes which have been described in the literature as austempering in which the quenched product is caused to transform from the high-temperature modification to another modification at selected temperatures thereby obtaining selected hardnesses and microstructures may be utilized in the performance of my process.

Some of the unique features of my process are the fact that I select shot of such a chemical composition that they may be either partially or completely malleableized and that the hardness thereof may be variably controlled by various quenching operations. An important factor is that I am able to obtain shot having various desirable properties by subjecting chilled cast -iron shot of selected materials and subjecting them to selected heat treating processes. Cast iron is a relatively. cheap material and yet my process permits of obtaining shot therefrom which possess more desirable properties than more expensive materials. Other characteristics of my new abrasive material will be evident from the appended claims.

Having thus described my invention, what I claim is:

1. A metal blasting material in the form of shot or grit, said material containing carbon in the range of 2.0 to 4%, silicon from .5 to 3%, manganese from .20 to 2%, sulfur f'romtraces to '.3%, phosphorus from .05 to 2% and the balance substantially iron, and said material having been at least partially malleableized from white iron and cooled at such a rate that its toughness is increased and its hardness is decreased to within a range from 125 to 500. on the Brinell scale.

2. A metal blasting material in the form of shot or grit, said material containing carbon in the range of 2.0 to4%, silicon from .5 to 3%, manganese from .20 to 2%, sulfur from traces to .3%, phosphorus from .05 to 2%, from traces to 1% of at least one carbide former selected from the group consisting of chromium, molybdenum,

vanadium, tungsten, zirconium, and titanium, and the balance substantially iron, and said ma:

terial having been at least partially malleableized from white iron and cooled at such a rate that its toughness is increased and its hardness is decreased to within a range from 125 to 500 on the Brinell scale.

s. A metal blasting material'in the mm of shot or grit, said material containing carbon in the range of 2.9 to 3.5%, silicon from 1.0 to 2.0%. manganese from .40 to .80%, sulfur from .05 to .15%, phosphorus from .10 to 1.0%, and the balanc substantially iron, and said material having been at least partially malleableized from white iron and cooled at such a rate that its toughness is increasedand its hardness is decreased to within a range from 125 to 500 on the Brinell scale.

4. A metal blasting material in the form of shot or grit, saidmaterial containing carbon in the range of 2.9 to 3.5%, silicon from 1.0 tov 2.0%, manganese from .40 to .80%, sulfur from .05 to .15%, phosphorus from .10 to 1.0%, from traces to 1% of at least one carbide former selected from the group consisting of chromium, molybdenum, vanadium, tungsten, zirconium, and titanium, and the balance substantially iron, and saidmaterial having been at least partially malleableized from white iron and cooled at such a rate that its toughness is increased and its hardness is decreased to within a range from 125 to 500 on the Brinell scale.

5. The method of producing metallic blasting material in the form of shotor grit, which comprises producing white iron particles of the desired form containing about 2.5 to 4% carbon, 0.5 to 3% silicon, .2 to 2% manganese, from traces to .3% sulfur, from .05 to 2% phosphorus and the balance substantially iron, heating said particles toa temperature from.1300 to 1600 F. for a period from one-half hour to six hours and cooling the heated particles, the said heating andthe rate of cooling being eflective to produce at least partial malleabilization, to impart thereto a hardness between 125 and 500 onthe Brinell scale and an improvement in toughness of at least 100%, as indicated by hammer tests on the material before and after heat treating.

6. The method of producing metallic blasting material in the form of shot or grit, which comprises producing white iron particles of the desired form containing about 2.5 to 4% carbon, 0.5 to 3% silicon, .2 to 2% manganese, from traces to .3% sulfur, from .05 to'2% phosphorus, from traces to 1% of a carbide former selected from the group consisting of chromium, molybdenum, vanadium, tungsten, zirconium and titanium, and the balance substantially iron, heating-said particles to a temperature from 1300 to 1600 F. for a period from one-half hour to six hours andcooling the heated particles, the said heatingand. the rate of cooling being effective to produce at least partial malleabilization, to impart thereto ahardness between 125 and 500 on the Brinell scale and an improvement in toughness of at'least 100%, as indicated by hammer tests on the material before

Images