Abstract:
A method for producing compacted, fully dense articles from atomized tool steel alloy particles by placing the particles in a deformable container, and isostatically pressing the particles at an elevated temperature to produce a precompact having an intermediate density. The precompact is heated to a temperature above the elevated temperature used to produce the precompact. The precompact is isostatically pressed to produce the fully-dense article.

Description:
This is a continuation-in-part application of patent application Ser. No. 09/003,368, filed Jan. 6, 1998, now U.S. Pat. No. 5,976,459. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for producing compacted, fully-dense articles from atomized, tool steel alloy particles by isostatic pressing at elevated temperatures. 
     2. Brief Description of the Prior Art 
     In the production of powder-metallurgy produced tool steel alloys by hot isostatic compaction, it is necessary to employ sophisticated, expensive melting practices, such as vacuum melting, to limit the quantity of non-metallic constituents, such as oxides and sulfides to ensure attainment of desired properties, such as bend-fracture strength, with respect to tool steel articles made from these alloys. Practices used in addition to vacuum melting to limit the non-metallic content of the steel include using a tundish or like practices to remove non-metallics prior to atomization of the molten steel to form the alloy particles for compacting, and close control of the starting materials to ensure a low non-metallic content therein. These practices, as well as vacuum melting, add considerably to the overall manufacturing costws for articles of thes type. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a method for producing compacted, fully-dense articles from atomized tool steel alloy particles that achieve final, compacted articles of reduced oxide content without resorting to the expensive prior art practices used for this purpose. 
     In accordance with the invention, a method is provided for producing compacted, fully-dense articles from atomized tool steel alloy particles that includes placing the atomized particles in an evacuated deformable container, sealing the container and isostatically pressing the particles within the sealed container at an elevated temperature to form a precompact. The elevated temperature may be up to 1800° F. or 1600° F. This pressing may be performed in the absence of prior outgassing of the powder-filled container. The precompact is heated to a temperature above the elevated temperature used to produce this precompact and is then isostatically pressed to produce the fully-dense article. The fully-dense article may have a minimum bend fracture strength of 500 ksi after hot working. 
     The heating of the particles to elevated temperature and/or the heating of the precompact may be performed outside of the autoclave that is used for the isostatic pressing. 
     The atomized tool steel alloy particles may be gas-atomized particles which may be nitrogen gas-atomized particles. 
     Prior to isostatic pressing, the tool steel alloy particles may be provided within a sealable container. This container is evacuated to provide a vacuum therein. In addition, the deformable container is evacuated to produce a vacuum therein. The alloy particles are introduced from the evacuated container to the evacuated deformable container through an evacuated conduit. The alloy particles are isostatically pressed within the deformable container at an elevated temperature to produce the precompact having an intermediate density. The precompact is heated to a temperature above the elevated temperature used to produce the precompact and the heated precompact is isostatically pressed to produce the fully-dense article. 
     &#34;Tool steel&#34; is defined to include high speed steel. 
     The term &#34;intermediate density&#34; means a density greater than tap density but less than full density (for example up to 15% greater than tap density to result in a density of 70 to 85% of theoretical density). 
     The term &#34;outgassing&#34; is defined as a process in which powder particles are subjected to a vacuum to remove gas from the particles and spaces between the particles. 
     The term &#34;evacuated&#34; means an atmosphere in which substantially all air has been mechanically removed or an atmosphere in which all air has been mechanically removed and replaced with nitrogen. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     By way of demonstration of the invention, a series of experiments was conducted using prealloyed powder. This powder, after mechanical sizing was placed in a container that was in turn connected to a deformable container through a vacuum connection. Both containers were independently evacuated, and then the powder was loaded by use of a vibratory feeder into the deformable container. After this container was filled, it was subsequently sealed and then consolidated. Consolidation was achieved by placing the container filled with powder into a pressure vessel having internal heating capability, sealing the pressure vessel, and simultaneously raising both the temperature and pressure in the vessel to a designated high value for each--typically about 2100° F. and 14,000 psi. This process is known as hot isostatic pressing (HIP). Another consolidation method (also HIP) is to heat the sealed container externally to the designated high temperature, transfer it to a pressure vessel, seal the pressure vessel, and raise the pressure quickly to the designated high value. The method of this invention involves a novel method of consolidation which is a two step process: (1) heating the loaded container to an elevated temperature and pre-compacting it to an intermediate density followed by (2) heating it to the high temperature and hot isostatically pressing it at the temperature and pressure parameters previously described. The elevated temperature for the pre-compaction step can be up to 1800° F. This pre-compaction step increases the density of the powder, but not to full density. 
     The tested alloys were designated as CPM 10V (10V), CPM M4 High Carbon (M4HC), and CPM M4 High Carbon with Sulfur (M4HCHS). 
     
                       TABLE 1______________________________________Composition of Alloys Tested (Balance Fe)Alloy   C      Mn     Si   S     Cr   Mo   W    V______________________________________10 V    2.45   0.50   0.90 0.07  5.25 1.30 --   9.75M4HC    1.40   0.30   0.30 0.05  4.00 5.25 5.75 4.00M4HCHS  1.42   0.70   0.55 0.22  4.00 5.25 5.75 4.00______________________________________ 
    
     All tests started with containers having a minimum diameter of 14 inches, and were conducted on material that had been hot worked with a reduction in area of at least 75%. M4 types were solution heat treated at 2200° F. and triple tempered at 1025° F. The data are presented by powder type, alloy, and consolidation method. The conventional consolidation method in which the temperature and pressure are simultaneously raised is designated as &#34;CCMD HIP.&#34; The process of externally heating, transferring to the pressure vessel, and raising the pressure is designated at &#34;CSMD HIP.&#34; The method of the invention as described in the preceding paragraph is designated as &#34;WIP/HIP.&#34; 
     Table 2 presents data from trials of the alloy designated as M4HCHS. The AW OFFICES practice used to produce this alloy powder comprised melting raw materials in an induction furnace, adjusting the chemistry of the molten alloy prior to atomization, pouring the molten alloy into a tundish with a refractory nozzle at the base of the tundish, and subjecting the liquid metal stream from that nozzle to high pressure nitrogen gas for atomization thereof, to produce spherical powder particles. 
     
                       TABLE 2______________________________________M4HCHS       Consol- Bend Fracture ResultsTrial             idation        Average                                  Max., Min.Number  Powder Size             Method    Tests                            (ksi) (ksi)______________________________________MFG 17  -16 Mesh  CCMD HIP   6   434   458,382MFG 18  -16 Mesh  CCMD HIP   6   475   530,433MFG 43  -16 Mesh  CCMD HIP   6   541   581,496MFG 44  -16 Mesh  CCMD HIP   5   548   594,488MFG 40  -35 Mesh  CCMD HIP   5   576   597,554MFG 41  -35 Mesh  CCMD HIP   6   534   605,380MFG 42  -35 Mesh  CCMD HIP   3   461   536,318MFG 69  -35 Mesh  CCMD HIP  15   617   674,567MFG 70  -35 Mesh  CCMD HIP  15   589   632,467MFG 61  -35 Mesh  CCMD HIP   6   506   570,455MFG 71  -35 Mesh  CCMD HIP  15   463   551,360MFG 72  -35 Mesh  CCMD HIP  12   455   550,361MFG 105 -35 Mesh  CCMD HIP  15   517   596,400MFG 106 -35 Mesh  CCMD HIP  15   484   583,441MFG 107 -35 Mesh  CCMD HIP  15   505   574,428MFG 108 -35 Mesh  CCMD HIP  13   506   596,405MFG 109 -35 Mesh  CCMD HIP  75   559   630,422MFG 73  -35 Mesh* CCMD HIP  15   454   530,228MFG 105A   -35 Mesh* CCMD HIP  15   543   579,496MFG 106A   -35 Mesh* CCMD HIP  15   495   565,418MFG 107A   -35 Mesh* CCMD HIP  15   449   530,393MFG 72  -35 Mesh**             CCMD HIP  15   467   527,386MFG 72  -35 Mesh**             CCMD HIP  14   459   600,350MFG 72  -35 Mesh**             CCMD HIP  15   450   543,330MFG 66  -35 Mesh  WIP/HIP   15   439   528/361MFG 67  -35 Mesh  WIP/HIP   15   429   541,299MFG 68  -35 Mesh  WIP/HIP   15   488   577,344MFG 69  -35 Mesh  WIP/HIP   15   597   645,525MFG 70  -35 Mesh  WIP/HIP   30   569   594,459MFG 105 -35 Mesh  WIP/HIP   15   466   539,253MFG 106 -35 Mesh  WIP/HIP   15   446   525,353MFG 107 -35 Mesh  WIP/HIP   15   404   504,245MFG 108A   -35 Mesh  WIP/HIP   29   448   562,322MFG 108B   -35 Mesh  WIP/HIP   30   443   518,269MFG 109 -35 Mesh  WIP/HIP   60   525   593,431______________________________________ -35 Mesh*: Finer than normal distribution. -35 Mesh**: Various mixtures of -35 mesh and -100 mesh powder. 
    
     As may be seen from the Table 2 data, product that was initially screened to -35 mesh and was consolidated by the CCMD HIP showed individual test results of bend fracture strengths up to 674 ksi. The averages ranged from a low of 449 ksi to a high of 617 ksi. The minimum bend fracture strength test results are not characteristics of the practice. These low results were caused by large exogenous inclusions present at the bend fracture surfaces. 
     The exogenous inclusions were identified as either slag or refractory particles. The slag originated from oxidized material as a result of exposure to air during melting. The refractory originated from erosion during the melting and the pouring of the alloy prior to atomization. They thus originated during melting and it is their presence that caused the low bend fracture results. 
     These low results are caused, therefore, not by the consolidation practice, but by the melting practice, and are not characteristic of the properties typically resulting from use of the consolidation practice. The maximum bend fracture strength of the product consolidated by the WIP/HIP method was 645 ksi, which is only slightly below the maximum value from the CCMD HIP. The average bend fracture strength values using WIP/HIP ranged from a low of 404 ksi to a high of 597 ksi. There is some difference between the CCMD HIP and the WIP/HIP process, but it is quite small. The low minimum values are caused by melting, not consolidation, so it is the high value of the averages that is most significant. Because productivity was much greater using the WIP/HIP process, and the capital equipment necessary to practice it costs much less than that required for CCMD HIP, there is an economic advantage to the method in accordance with the invention. Both the maximum values and the average bend fracture strengths of the two consolidation methods are comparable. These data clearly show that the WIP/HIP consolidation method yielded high bend fracture strength results. 
     A smaller number of trials was run on M4HC produced by the same practice as used in the production of M4HCHS. Results from these trials are shown in Table 3. 
     
                       TABLE 3______________________________________M4HC       Consol- Bend Fracture ResultsTrial             idation        Average                                  Max., Min.Number  Powder Size             Method    Tests                            (ksi) (ksi)______________________________________MFG 33  -35 Mesh  CCMD HIP  6    622   666,589MFG 34  -35 Mesh  CCMD HIP  6    606   647,581MFG 35  -35 Mesh  CCMD HIP  6    622   639,577No Number   -35 Mesh  CCMD HIP  6    708   732,658MFG 36  -35 Mesh  CCMD HIP  6    612   627,595MFG 37  -35 Mesh  CCMD HIP  6    615   653,550MFG 38  -35 Mesh  CCMD HIP  4    663   695,607MFG 73  -35 Mesh* CCMD HIP  15   454   530,228MFG 37  -35 Mesh* WIP/HIP   3    580   615,493______________________________________ 
    
     Two observations can be made: (1) the bend fracture strength of the lower sulfur (M4HC) material was significantly greater than for the high sulfur (M4HCHS) material, regardless of the consolidation method, and (2) the average bend fracture strength of the WIP/HIP material, while well above 500 ksi, was below that consolidated by CCMD HIP. 
     Table 4 shows the data from trials of 10V alloy produced by the same practice as M4HCHS. 
     
                       TABLE 4______________________________________10 V       Consol- Bend Fracture ResultsTrial             idation        Average                                  Max., Min.Number  Powder Size             Method    Tests                            (ksi) (ksi)______________________________________MFG 7   -35 Mesh  CCMD HIP  48   572   651,331MFG 8   -35 Mesh  CCMD HIP  48   578   651,357MFG 45  -35 Mesh  CCMD HIP  18   562   656,348MFG 46  -35 Mesh  CCMD HIP  18   563   644,361MFG 47  -35 Mesh  CCMD HIP  12   550   640,386MFG 48  -35 Mesh  CCMD HIP  12   558   645,402MFG 52  -35 Mesh  CCMD HIP  12   602   649,551MFG 53  -35 Mesh  CCMD HIP  24   615   663,552MFG 55  -35 Mesh  CCMD HIP  11   616   663,552MFG 61  -35 Mesh* CCMD HIP  12   587   663,552MFG 63  -35 Mesh* CCMD HIP  15   550   621,385MFG 65  -35 Mesh* CCMD HIP   3   610   646,592MFG 63  -35 Mesh* WIP/HIP   20   540   612,409MFG 49  -35 Mesh  CSMD HIP   6   456   523,405______________________________________ 
    
     These results show that WIP/HIP consolidation gave average bend fracture strengths for this alloy that are lower than the CCMD HIP consolidation, but significantly above the CSMD HIP. The values below 500 ksi with the CCMD HIP or WP/HIP consolidation had large exogenous inclusions in the fracture surface, as a result of the melting practice. The maximum strength values showed that the WIP/HIP method gave strengths about 50 ksi lower than CCMD HIP, but still well above the 500 ksi minimum. 
     All of the WIP/HIP trials discussed above used a temperature of 1400° F. for the pre-compacting temperature. This temperature was chosen based on work that is described hereafter. In all of the above disclosed cases, the loaded compacts were externally heated and transferred to the pressure vessel and the pressure was quickly raised to 11,000 psi. After this pre-compaction step, the compacts were each transferred to a furnace operating at 2150° F., equalized, and then transferred to the pressure vessel. 
     The vessel was sealed and quickly pressurized to 14,000 psi. The consolidated compacts, regardless of the consolidation method, were all thermo-mechanically processed to about 85% reduction from their original size before the bend fracture strength was tested. 
     Experimental work was carried out on the effect of heating at various temperatures prior to conventional consolidation (CCMD HIP). M4HCHS powder screened to -35 mesh was loaded into 5&#34; diameter cans, sealed, and heated for five hour at temperatures ranging from 1400 to 2185° F. After holding at this temperature, the compacts were given conventional (CCMD HIP) consolidation with final temperature and pressure of 2185° F. and 14,000 psi, respectively. Bend fracture strength tests were run in the as-HIP condition, and after hot working with an 82% reduction in area from the original compact size. Test results are given in Table 5. 
     
                       TABLE 5______________________________________Bend Fracture Test Results on Pre-Heated Powder Pre-Heated   As-HIP Bend                         Hot-Worked BendPowder Temperature  Fracture   FractureSource (° F.)              (ksi)      (ksi)______________________________________A     No Hold      492        603 1400         501        602 1600         452        605 1800         453        601 2000         429        579 2185         367        582B     No Hold      529        647 1400         547        643 1600         426        642 1800         446        601 2000         405        578 2185         362        567______________________________________ 
    
     These results show that when unconsolidated powder was held at temperatures above 1400° F., bend fracture strengths in the as-HIP condition were lowered, When tested after an 82% reduction by hot working, bend fracture strengths were not lowered until the powder is held at temperatures in excess of 1600° F. As a result of these data, all heating for the pre-compaction was done at 1400° F. as previously stated. 
     To determine the reason for this degradation in bend fracture strength, a determination had to be made as to whether heating at these different temperatures has any effect on the sulfide and oxide distribution, both in the as-HIP condition and after hot working. The results of this examination are given in Table 6. 
     
                       TABLE 6______________________________________Sulfide Distribution on Pre-Heated Powder Pre-Heat   Sulfide Distribution                          Sulfide DistributionPowder Temperature            As-HIP        Hot WorkedSource (° F.)            Area    Max. Size                            Area  Max. Size______________________________________B     No Hold    225     3.61    253   6.56 1400       152     2.59    124   5.85 1600       185     3.38    343   13.34 1800       315     4.19    402   5.76 2000       540     5.06    656   9.43 2185       993     10.78   1071  18.53______________________________________ 
    
     These data show that if the pre-heat temperature is 1600° F. or higher, the total sulfide area increased, and the increase was greater with a higher hold temperature. This is shown for both as the as-HIP as well as the hot worked condition. It is well known that larger inclusions as well as larger total area of inclusions cause a decrease in bend fracture strength. Microstructural examination of the effect of pre-heat temperature on oxide growth showed no apparent increase in the size of the oxides for pre-heat temperatures up to 2000° F. but at pre-heat temperatures above 1600° F. there was a noticeable outlining of the prior particle boundaries indicating the beginning of an increased concentration of oxides. For these reasons, all production trial compacts were pre-heated at 1400° F. but could have been pre-heated up to 1600° F. without any detrimental affect. 
     
                       TABLE 7______________________________________M4HCHS       Consol- Bend Fracture StrengthTrial             idation        Average                                  Max., Min.Number  Powder Size             Method    Tests                            ksi   ksi______________________________________HIP 1   -16 Mesh  CCMD HIP   5   388   455,336HIP 1   -35 Mesh  WIP/HIP    6   368   415,305MFG 110 -35 Mesh  WIP/HIP   30   419   519,262MFG 111 -35 Mesh  WIP/HIP   15   417   476,342______________________________________ 
    
     Four trials were performed in which M4HCHS prealloyed powder was loaded in air into a-deformable container, without the container being previously or subsequently evacuated. This practice is termed as &#34;air loading.&#34; After air loading, the container was sealed and then consolidated. The consolidation practices employed were as earlier described as &#34;CCMD HIP&#34; and in accordance with the invention described as &#34;WIP/HIP.&#34; The results of these trials are presented in Table 7. 
     Comparison of the data from the three WIP/HIP trials with the data from the CCMD HIP trial shows that the average Bend Fracture Strength test results are comparable for the two different consolidation practices employed. Two of the WIP/HIP trials produced maximum values for Bend Fracture Strength exceeding the maximum value for the CCMD HIP trial. In all of these trials the Bend Fracture Strength values were degraded by the presence of exogenous inclusions detected on the fracture surfaces. These inclusions resulted from refractory contact during melting of the alloy from which the prealloyed powder particles were produced. 
     Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.