DE69419474T2 - Articles made of tool steel powder containing sulfur and process for its manufacture - Google Patents

Abstract

A powder-metallurgy produced tool steel article of a hot worked, fully dense, consolidated mass of prealloyed particles of a tool steel alloy having a sulfur content within the range of 0.10 to 0.30 weight percent and a maximum sulfide size below about 15 microns. <IMAGE> <IMAGE>

Description

Scope of the invention

The invention relates to a tool steel article made from a hot-worked powder metallurgical tool steel having a higher than usual sulfur content and a method for producing this article.

Description of the state of the art

Tool steels are commonly used in the manufacture of tools used in both cutting and non-cutting tool applications. This includes the manufacture of broaches and cutters as well as rolls, punches and die components. In these tool applications, it is necessary that the tool steel possess sufficient strength, toughness and wear resistance to withstand the service conditions encountered in these typical applications. In addition, they must have adequate machinability and grindability to facilitate the manufacture of the desired tool components.

It is known that the presence of sulphur in tool steels improves their machinability and grindability by forming sulphides which act as a lubricant between the cutting tools used to manufacture the tool components and the chips which are removed from the steel during this process. The sulphides also promote the breaking of the chips during the cutting process in tool manufacturing, thereby further facilitating this process.

It is known that the use of sulfur in amounts greater than 0.10% reduces the hot workability of conventional ingot cast tool steels and adversely affects their mechanical properties, particularly their toughness. In conventional tool steels with high sulfur content, the sulfides are typically larger and elongated in the direction of hot working. Similarly, in conventional wrought tool steels, the primary carbides in the steel are elongated during hot working so that they form carbide threads in the direction of machining. The carbide threads in these steels adversely affect the mechanical properties and their negative effects are so pronounced that they generally outweigh any adverse effects of the sulfides in this respect.

On the other hand, when tool steel articles with higher sulfur content are produced by powder metallurgy processes, in which pre-alloyed particles of the steel are consolidated to produce a completely dense article, the carbides are relatively small and well distributed compared to those of conventional tool steels. Because of the favorable size and distribution of the carbides achieved in these tool steels, the adverse effects of the carbide threads encountered in conventional wrought steels are avoided. The properties of the tool steels produced by powder metallurgy are therefore more responsive to changes in the sulfur content and in the size and distribution of the sulfides introduced for the purpose of improving their machinability or grindability. For this reason, sulfur contents of more than about 0.07% are not generally used in powder metallurgy tool steels because the sulfides have adverse effects on their mechanical properties, as indicated, for example, by a decrease in the bending strength of the steel. Powder metallurgy tool steel articles with higher sulfur contents would be used to a greater extent if the detrimental effects of sulfur on their mechanical properties could be avoided.

Summary of the invention

Thus, it is a primary object of the present invention to provide a tool steel article made from a hot worked high sulfur tool steel produced by a powder metallurgy process, wherein the presence of sulfur and the resulting sulfides does not significantly adversely affect the mechanical properties, while a beneficial effect in terms of improved machinability and grindability is achieved.

A more specific object of the invention is to provide a tool steel article made from a hot worked, powder metallurgical tool steel having a higher sulfur content, wherein the presence of sulfur and the resulting sulfides does not appreciably degrade toughness as characterized by flexural rupture strength.

The invention provides a machinable, powder metallurgically produced, sulfur-containing tool steel article according to claims 1 or 2 of the appended claims.

Generally, according to the invention, a machinable, powder metallurgically produced, sulfur-containing tool steel article is made which contains a hot worked, fully dense, consolidated mass of prealloyed, nitrogen gas atomized particles of a tool steel alloy having a sulfur content of 0.10 to 0.30 weight percent with a maximum sulfide size controlled to be less than is approximately 15 µm.

The tool steel alloy of the hot worked article may have the composition of a forged high speed steel or a forged cold worked tool steel to which sulfur has been intentionally added in a range of 0.10 to 0.30 weight percent. In general, the tool steel of the hot-worked article may contain, in weight percent, 0.80 to 3.00 carbon, 0.20 to 2.00 manganese, 0.10 to 0.30 sulfur, up to 0.04 phosphorus, 0.20 to 1.50 silicon, 3.00 to 12.00 chromium, 0.25 to 10.00 vanadium, up to 11.00 molybdenum, up to 18.00 tungsten, up to 10.00 cobalt, up to 1.10 nitrogen and up to 0.025 oxygen, with the balance being iron and incidental impurities. Tungsten may be substituted for molybdenum in a stoichiometric ratio of 2:1.

The machinable article made from sulfur-containing tool steel by a powder metallurgy process may have a minimum transverse bending strength of 3450 MPa (500 ksi) when heat treated to a hardness of 64 to 66 HRC. The article comprises a hot worked, fully dense, consolidated mass of prealloyed, nitrogen gas atomized particles of a tool steel alloy comprising, by weight, 1.25 to 1.50 carbon, 0.20 to 1.00 manganese, 0.10 to 0.26 sulfur, up to 0.04 phosphorus, up to 1.00 silicon, 3.0 to 6.0 chromium, 4.0 to 6.0 molybdenum, 3.50 to 4.50 vanadium, 4.0 to 6.5 tungsten, up to 0.025 oxygen, and up to 1.10 nitrogen, with the balance being iron and incidental impurities. The article has a maximum sulfide size adjusted to be less than about 15 µm.

Preferably, the sulfur content of the articles according to the invention may be in the range from 0.14% to 0.26%.

The invention further provides a method for producing a powder metallurgical, sulfur-containing tool steel article according to claims 4 or 5 of the appended claims.

The invention comprises a process for producing a powder metallurgical sulfur-containing tool steel article from a hot worked, fully dense, consolidated mass of prealloyed, nitrogen atomized particles of a tool steel alloy having a sulfur content of 0.10 to 0.30 weight percent with a maximum sulfide size adjusted to be less than about 15 microns. According to this process, prealloyed particles are produced by nitrogen gas atomization and hot isostatically densified to full density at a temperature of 1185°C (2165°F) and a pressure of 1035 MPa (15 ksi). The resulting densified body is hot worked at a temperature of 1121°C (2050°F) to achieve a desired article shape and the article is then stress relieved.

The process according to the invention can also be applied to prealloyed particles of a tool steel alloy having the following composition in weight %: 0.80 to 3.00 carbon, 0.20 to 2.00 manganese, 0.10 to 0.30 sulfur, up to 0.04 phosphorus, 0.20 to 1.50 silicon, 3.0 to 12.0 chromium, 0.25 to 10.0 vanadium, up to 11.0 molybdenum, up to 18.0 tungsten, up to 10.0 cobalt, up to 0.10 nitrogen and up to 0.025 oxygen, the balance being iron and incidental impurities.

The method according to the invention can also be used in conjunction with prealloyed particles of a tool steel alloy which comprises in weight percent the following components: 1.25 to 1.50 carbon, 0.20 to 1.00 manganese, 1.10 to 0.26 sulfur, up to 0.04 phosphorus, up to 1.00 silicon, 3.0 to 6.0 chromium, 4.0 to 6.0 molybdenum, 3.50 to 4.50 vanadium, 4.0 to 6.5 tungsten, up to 0.025 oxygen and up to 0.10 nitrogen, the remainder consisting of iron and incidental impurities.

Preferably, the sulfur content may be in the range of 0.14 to 0.26% by weight.

According to the invention, the carbon present in the alloy combines with chromium, vanadium, molybdenum and tungsten to form the desired distribution of wear-resistant carbides and to promote secondary hardening. Sufficient carbon is also present to provide strengthening of the steel matrix. The sulfur present in the steel combines primarily with the manganese to form manganese sulfides or manganese-rich sulfides which facilitate or enable the machinability and grindability of the steel.

In order to achieve the properties required for the powder metallurgically produced tool steel articles according to the invention, it is essential that the powder metallurgically produced, high sulfur tool steels used to make them are hot worked after consolidation to achieve the high mechanical strength required for tool components. It is also essential that the manufacturing and processing conditions for the powder metallurgically produced tool steels used in the articles according to the invention are controlled or adjusted so that the sizes and distribution of the sulfides induced by the addition of the sulfur do not appreciably deteriorate the mechanical properties. In the powder metallurgically produced tool steel used in the tool steel articles according to the invention, this is achieved by keeping the maximum size of the sulfides in their greatest extent below about 15 µm.

Detailed description of preferred embodiments

To demonstrate the invention, a series of experimental tool steels with varying sulfur content were produced and subjected to various tests for their mechanical properties and machinability. Mu sters of several commercial powder metallurgical high speed steels were subjected to the same tests for comparison purposes. With the exception of the sulfur content, the commercial powder metallurgical tool steels have the same nominal composition as the experimental tool steels. The actual chemical compositions of the experimental tool steels and the commercially produced tool steels are shown in Tables I and II. Table I Chemical composition of experimental powder metallurgical tool steels Table II Chemical composition of commercial tool steels with high sulfur content

The conditions of manufacture for the experimental tool steels were chosen to minimize the size of the sulfides in the microstructure. They were produced from nitrogen gas atomized prealloyed powders prepared from 136 kg (300 pounds) of induction melted heats. Approximately 91 kg (200 pounds) of powder from each heat was sieved to a fineness of -16 mesh (US standard) and placed in 20 cm (8 inch) diameter low carbon steel containers heated at 204ºC (400ºF). hot degassed and then sealed by welding. The containers were then heated to 1185ºC (2165ºF), isostatically densified at that temperature for four hours at a pressure of 103.5 MPa (15 ksi), and then slowly cooled to ambient temperature. The resulting densified bodies were then heated to a temperature of 1121ºC (2050ºF), hot worked into 8 cm (3 inch) diameter bars, and finally stress relieved using a conventional high speed tool steel normalizing cycle.

The conventional powder metallurgy tool steels were made from nitrogen atomized powders of -16 mesh and are typical of materials that have undergone varying degrees of hot reduction after consolidation by isostatic pressing. No special measures were taken in the manufacture of these steels to control or adjust the sulfide size.

Several tests were conducted to compare the properties of the tool steel articles of the invention with those of articles made from high sulfur powder metallurgy tool steels by other manufacturing processes. Tests were conducted to demonstrate the effects of composition and manufacturing processes on sulfide size, transverse rupture strength, notch strength and machinability. The machinability tests were conducted on samples in the fully stress relieved annealed condition, while the transverse rupture and notch tests were conducted on samples in the hardened and tempered condition. The heat treatment for the latter samples included austenitizing for 4 minutes in molten salt at 1024°C (2200°F), oil quenching to room temperature and triple tempering in molten salt for two hours plus two hours plus two hours at 552°C (1025°F). After this heat treatment, the hardness of the samples was between 64 and 66 Rockwell C. The sizes and distribution of the sulfides in the experimental and commercial tool steels are shown in Figs. 1 and 2, respectively. As expected, the number of sulfides in the experimental tool steels increases with sulfur content, as can be seen by comparing the microstructures for steels 92-17, 92-18, 92-19 and 92-20 in Fig. 1. It is also clear that according to the invention, all sulfides in the experimental tool steels, regardless of their sulfur content, are less than 15 µm in their longest dimension. It is also clear that the size of the sulfides in the experimental tool steels in their longest dimension is considerably smaller than that of the sulfides in the commercial tool steels of similar composition. As shown in Fig. 2, the size of the sulfides in the latter steels varies in a length range of 20 µm to 30 µm depending on the hot reduction carried out during production.

The Charpy C-notch properties and flexural strengths of the experimental and commercial tool steels are shown in Tables III and IV, respectively. A comparison similar to the results for the experimental tool steels shows that by keeping the maximum sulphide size below 15 µm it is possible to increase the sulphur content for the purpose of improving machinability without sacrificing toughness. This is indicated by the fact that the notch strength and transverse rupture strength of the experimental steels are essentially equivalent in both the longitudinal and transverse directions for sulphur contents in the range 0.005 to 0.26%. Table III Notch strength and transverse rupture strength of experimental tool steels1) Table IV Notch strength and bending strength of commercially available tool steels1)

¹ (austenitized at 1024ºC (2200ºF) for 4 minutes, oil quenched and triple tempered at 552ºC (1025ºF) for 2 plus 2 hours)

A comparison of the mechanical properties presented for commercial tool steels in Table IV shows that their notch strength and transverse rupture strength are generally improved by increasing the extent of hot reduction, although this results in some elongation of the sulfides. However, because of the greater extent of the sulfides in these steels, their mechanical properties are significantly worse than those of the experimental tool steels having essentially the same composition and the same extent of hot reduction. Compare, for example, the mechanical properties of steel 92-20 (0.26% S), which has a maximum sulfide size of about 15 µm and longitudinal and transverse rupture strengths of 5320 MPa (771 ksi) and 3871 MPa (561 ksi), respectively, with those of steel 92-78 (0.24% S) with a maximum sulfide size of approximately 30 µm and longitudinal and transverse bending strengths of 4492 MPa (651 ksi) and 2643 MPa (383 ksi), respectively.

The results of the drilling machinability tests conducted on the experimental tool steels in the stress-relieved condition are shown in Table V. The drilling machinability indices in this table were obtained by comparing the times required to drill holes of the same size and depth in these steels and multiplying the ratios of these times for each steel to the time for the experimental steel having a sulfur content of 0.005% by 100. Indices greater than 100 indicate that the drilling machinability of the steel tested is greater than that of the experimental tool steel article containing 0.005% sulfur (Steel 91-60). The results show that an increase in sulfur from 0.005% to 0.26% improves the machinability of the experimental tool steels and that the greatest improvement is achieved at a sulfur content of about 0.14% or above. Table V Influence of sulphur content on the drilling machinability of experimental tool steels

¹Drilling machinability index = (Time required to drill the test material/Time required to drill the reference standard material Time) · 100

From the above table it can be seen that by reducing the size of the sulphides in the articles made from hot worked powder metallurgical tool steels it is possible to substantially avoid the negative effects of the high sulphur content on their properties. Thus, according to the invention it is possible to produce powder metallurgically produced tool steel articles with a sulphur content higher than that normally permitted in order to achieve improved machinability without any appreciable deterioration in the mechanical properties as demonstrated in particular by the flexural strength of the steel.

The term "sulfur-containing tool steel article" is limited to cold worked tool steels and high speed tool steels.

Claims (6)

1. A machinable, powder-metallurgically produced, sulphur-containing tool steel article comprising a hot-worked, fully dense, consolidated mass of prealloyed particles of a tool steel alloy atomised with gaseous nitrogen, said tool steel alloy containing in weight percent 0.80 to 3.00 carbon, 0.20 to 2.00 manganese, 0.10 to 0.30 sulphur, up to 0.04 phosphorus, 0.20 to 1.50 silicon, 3.0 to 12.00 chromium, 0.25 to 10.00 vanadium, up to 11.00 molybdenum, up to 18.00 tungsten, up to 10.00 cobalt, up to 0.10 nitrogen, up to 0.025 oxygen and the remainder being iron and incidental impurities, characterized in that the article has a maximum sulfide size controlled to be less than 15 µm. 2. A machinable powder metallurgically produced sulfur-containing tool steel article having a minimum transverse bending ultimate strength of 3450 MPa (500 ksi) when heat treated to achieve a hardness of 64 to 66 HRC, said article comprising a hot worked, fully dense, consolidated mass of gaseous nitrogen atomized prealloyed particles of a tool steel alloy containing, by weight percent, 1.25 to 1.50 carbon, 0.20 to 1.00 manganese, 1.10 to 0.26 sulfur, up to 0.04 phosphorus, up to 1.00 silicon, 3.0 to 6.00 chromium, 4.0 to 6.0 molybdenum, 3.50 to 4.50 vanadium, 4.0 to 6.5 tungsten, up to 0.025 oxygen, up to 0.10 nitrogen, the balance being iron and incidental impurities, characterized in that the article has a maximum sulfide size controlled to be less than about 0.15 µm. 3. A powder metallurgically produced sulfur-containing tool steel article according to claim 1 or 2, wherein the sulfur content is in the range of 0.14 percent to 0.26 percent. 4. A process for making a powder metallurgical sulfur-containing tool steel article comprising a hot worked, fully dense, consolidated mass of nitrogen atomized prealloyed particles of a tool steel alloy comprising, by weight percent, 0.80 to 3.00 carbon, 0.20 to 2.00 manganese, 0.10 to 0.30 sulfur, up to 0.04 phosphorus, 0.20 to 1.50 silicon, 3 to 12.00 chromium, 0.25 to 10.00 vanadium, up to 11.00 molybdenum, up to 18.00 tungsten, up to 10.00 cobalt, up to 0.10 nitrogen and up to 0.025 oxygen, the balance being iron and incidental impurities, and wherein the article has a maximum sulfide size controlled so as to is less than about 15 µm, and the process comprises the steps of preparing the prealloyed particles by atomization with gaseous nitrogen, hot isostatically compacting the prealloyed particles to full density at a temperature of 1185ºC (2165ºF) and a pressure of 103.5 MPa (15 ksi), and hot working the resulting compacted body to the desired article shape at a temperature of 1121ºC (2050ºF), and annealing the article. 5. A process for producing a powder metallurgical sulfur-containing tool steel article having a minimum transverse bending rupture strength of 3450 MPa (500 ksi) when heat treated to achieve a hardness of 64 to 66 HRC, said article comprising a hot worked, fully dense, consolidated mass of nitrogen atomized, prealloyed particles of a tool steel alloy containing, by weight percent, 1.25 to 1.50 carbon, 0.20 to 1.00 manganese, 0.10 to 0.26 sulfur, up to 0.04 phosphorus, up to 1.00 silicon, 3.0 to 6.0 chromium, 4.0 to 6.0 molybdenum, 3.5 to 4.50 vanadium, 4.0 to 6.5 tungsten, up to 0.025 oxygen, up to 0.10 nitrogen, the balance being iron and incidental impurities, and wherein said article has a maximum sulfide size controlled to be less than about 15 µm, and the process comprising the steps of preparing prealloyed particles by nitrogen gas atomization, compacting the prealloyed particles to full density at a temperature of 1185ºC (2165ºF) and a pressure of 103.5 MPa (15 ksi), hot working the compacted body at a temperature of 1121ºC (2050ºF) to achieve the desired shape of the article, and annealing the article. 6. A process according to claim 4 or 5, wherein the sulfur content is in a range of 0.14 to 0.26 percent by weight.