Abstract:
The following specification describes a process for improving the hardness and other mechanical properties of iron and steel Powder Metallurgy (P/M) parts. The first stage of the novel process consists of heating to and holding at a temperature between 590° C. to 720° C. unalloyed or low alloyed P/M parts in an atmosphere containing a Nitrogen donor such as Ammonia in either batch or continuous furnaces. The concentration of ammonia during the first stage is maintained between 3% to 15%. The second stage of the inventive process is an ‘aging’ process which may be conducted either as an in-line process or as a stand-alone independent process that involves the heating of P/M parts that have fully or partially cooled after the first stage to a temperature between 180° C. and 660° C. in an atmosphere of plain air or Nitrogen. The first stage may be performed in varying concentrations of the nitrogen donor wherein the temperature and time duration may also be varied to control the depth of hardening in the said part. The conditions may be optimized to achieve through hardness of the part without embrittllement. The optional stage two of the technology is an aging process that does not involve “quenching,” thereby significantly lowering distortion of treated parts and eliminating pollution associated with liquid quenching. The technology improves process economy by using low temperatures and consequently fuel consumption.

Description:
[0001]    The following specification describes a process for improving the hardness and other mechanical properties of iron and steel Powder Metallurgy (P/M) parts in which the first stage is the alloying of the parts with Nitrogen which causes the formation of an austenitic phase in the metal matrix of the parts in addition to the formation of hard transformation products and interstitial Nitrogen throughout the section thickness of the parts or to a substantial depth below the surface of the parts depending on process parameters employed such as temperature, time at temperature, composition of the atmosphere gas mixture and the properties of the P/M parts such as density, thickness and alloying elements. 
         [0002]    This specification also describes a subsequent second stage of “aging” the P/M parts which converts the bulk of the above mentioned austenitic phase to hard transformation products which are crystalline structures including but not restricted to structures commonly referred to as bainite&amp;martensite. This increases the hardness of P/M parts. The strength of P/M parts with a lower carbon content increases while the strength of parts with higher carbon content reduces in which case the second stage of the process is performed only in applications where the strength of the P/M part is not primary to its application. The second stage is a novel aging process wherein the tradition quenching process has been eliminated to reduce part distortion among other benefits such as lowering of environmental pollution. 
       FIELD AND USE OF INVENTION 
       [0003]    Several iron &amp; steel components and parts in various areas of application which include automotive, machinery, hardware, sports, firearms and domestic appliance are made by the powder metallurgy route (these being known as P/M parts) as the cost of production is lower than other production processes. However P/M parts are inherently porous and this makes them less suitable for use in stressed applications unless they are hardened and strengthened in a variety of ways which include alloying with expensive elements, increasing part hardness and density by the process of heat treatment, impregnation and by mechanical working. Each of these processes has some disadvantages which include additional cost, environmental pollution and part distortion. 
         [0004]    The said inventive process is a method of hardening and strengthening P/M parts by gas alloying and this process does so in a manner that reduces or eliminates several disadvantages of traditionally practiced processes. This novel process offers a radically different approach to hardening of P/M parts either as a conjunct process to the prior sintering process in which the P/M parts are actually manufactured or as an independent stand alone process. 
         [0005]    This process has the benefits of lower energy consumption, lower pollution, less shape distortion and reduction in the use of expensive alloying elements. 
       PRIOR ART 
       [0006]    Iron and steel P/M parts are widely employed in automotive, machinery, hardware, sports, firearms and domestic appliances. Powder metallurgy is a manufacturing method where metal powders are compacted in a die cavity that is nearly the shape of the final product prior to being “sintered” during which the powders form a bond, get consolidated into a single monolith and shrinks to the final shape with least machining requirement. The powder metallurgy manufacturing process offers many advantages which cannot be attained by other metalworking processes such as highest raw material utilization, least machining, high quality consistency and lowest manufacturing costs for parts where P/M is a feasible manufacturing process. 
         [0007]    Due to the nature of the manufacturing process, P/M parts do not attain full density, have porosity and are consequently unsuitable for stressful applications. The strength &amp; hardness of P/M parts can be improved by heat treatment processes which are well known in the art. 
         [0008]    Heat Treatment of P/M parts is no different to the heat treatment of wrought parts, whether forged, stamped or cast and then machined and is done by heating them in a controlled atmosphere to a temperature slightly higher than the upper transformation temperature of the alloy the part is made of, usually above 800° C. followed by rapid cooling, normally by quenching in a liquid, usually oil, to obtain a hard martensitic and/or bainitic structure in the case of parts with adequate Carbon content. The quenched parts are then tempered to reduce brittleness. 
         [0009]    In the case of low alloyed and unalloyed low P/M low carbon parts the process of hardening as generally described above is performed but in an atmosphere that donates Carbon, such process being called Carburising and in several cases with the addition of Nitrogen, such process being called Carbonitriding, both of which are referred to as case hardening processes as only the surfaces of the treated parts, to the depth of diffusion of Carbon/Nitrogen is hardened, the core being softer. 
         [0010]    The surface treatmented P/M parts are quenched usually in oil and there is a need to remove the oil from the surface and interior of the P/M parts and this is an expensive and polluting process involving either washing in a heated liquid detergent medium which generates polluting effluent or by burning which causes air pollution. Advanced processes that reduce pollution such as vacuum de-oiling add significant cost to the process. 
         [0011]    Of more recent origin is the use of vacuum furnaces for hardening, carburizing and carbonitriding of P/M parts where quenching is done by recirculation and cooled gas under pressure. While this process eliminates the pollution of oil quenching it nonetheless is an expensive plant and an extra manufacturing step. Additional cost is incurred by the necessity of having to use expensive alloying elements in the P/M part so the desired hardness can be obtained by gas quenching. 
         [0012]    The heat extraction from the P/M part while quenching depends upon among other factors, the part section thickness. Non uniform cooling of a part with thick as well as thin sections results in dimensional distortion due to non-uniform cooling. 
         [0013]    Nitriding &amp; Nitrocarburising are surface treatments which do not normally involve quenching except in some cases. The difficulty of applying these processes to P/M parts is that a defined “case depth” is not easily obtained due to the porous nature of P/M parts which causes non uniform penetration the remedies to which are either an additional prior process of porosity sealing or the relatively more expensive process of plasma nitriding. 
         [0014]    Sinter hardening is another hardening process where the parts are rapidly cooled as they emerge from a continuous atmosphere sintering furnace by the impingement of re-circulated &amp; cooled furnace atmosphere. However as the heat extraction characteristics of gas is significantly lower than that of liquid quenchants, the P/M parts have to be sufficiently alloyed with expensive elements such as Nickel &amp; Molybdenum which increase the hardenability of the part but also make the part more expensive. 
         [0015]    As is evident, all known hardening techniques in industrial practice suffer from a variety of disadvantages and there is present a need for an improved technique which is the subject matter of the said inventive process technology. 
         [0016]    The technology described herein in detail, offers a solution that has never been attempted by other workers in the area of powder metallurgy familiar with the art. It is a completely different genre when it comes to treatment of P/M parts for improved strength and hardness. A thorough search of patent literature has not found any document that matches the disclosure either in concept, content or spirit. The closest reference was found in a scientific paper by X, Yang &amp; C. Kong &amp; Y. Uiao entitled “A study on austenitic nitrocarburising without compound layer” presented at 2nd International Conference of Carburizing and Nitriding [ Proceedings of the second International Conference on Carburizing and Nitirding with Atmoshphere, 6-8 Dec. 1995, Cleveland, Ohio]. However this paper pertains only to the processing of wrought parts, not P/M parts. Additionally the paper describes only a conventional shallow surface hardening process &amp; not a process where the part is hardened throughout its section thickness or to a substantial depth below the part surface. It also does not describe the improvement in properties which result from subsequent aging. 
         [0017]    The technology described in its entirety in the subsequent sections will amply disclose the novel features and the intended utility of this unique hardening method developed for use on P/M parts. 
       OBJECT OF THE INVENTION 
       [0018]    The principal object of the invention is to devise a hardening and strengthening process suitable for all types of iron &amp; steel P/M parts including those with little or no hardenability enhancing alloying elements. 
         [0019]    Another objective is to achieve hardness and strength by the formation of nitrogen rich austenite in the metal matrix of the P/M parts either throughout their section thickness or to a substantial depth below the part surface. 
         [0020]    Another object of the invention is to exclusively create said nitrogen rich austenite in the metal matrix of the P/M part where subsequent aging of the processed part is optional and may be eliminated where aging is found to be unnecessary. 
         [0021]    Another objective is to ‘age’ the P/M parts to convert the Nitrogen rich austenite to hard transformation products for improved hardness and strength. 
         [0022]    One more objective of “aging” is to harden parts with a lower carbon content for internal strength and hardness and an increase in hardness without an increase in strength of parts with a higher carbon content. 
         [0023]    Yet another object of the invention is to make negligible the formation of embrittlling iron nitrogen compounds when performing the said novel process. 
         [0024]    Yet another object of the invention is to create iron nitrogen compounds on the surface when performing the said novel process where required without causing embrittlment. 
         [0025]    One more object of the invention is to improve the hardness and mechanical properties of P/M parts subjected to the said novel process without rapid cooling or quenching 
         [0026]    Another object of the invention is to foster versatility to the said novel inventive process by enabling performance of the process in continuous or batch furnaces or in furnaces that work at or above atmospheric pressure as well as vacuum furnaces. 
         [0027]    Another object of the invention is to provide a versatile process that may either be performed in a module attached to the furnace or may be performed in a furnace attached to or in line with another furnace. 
         [0028]    Another object of the invention is to reduce the distortion of P/M parts. 
         [0029]    Another object of the invention is to lower the amount of energy involved in the hardening of P/M parts by the use of low process temperatures and consequently reduces costs. 
         [0030]    Another object of the invention is to lower the amount of environmental pollution involved in the hardening of P/M parts. 
       STATEMENT OF THE INVENTION 
       [0031]    Accordingly, the invention provides a novel process for increasing the hardness and strength of unalloyed and low alloyed P/M parts that is characterized by the use of low temperature gas alloying in an Ammonia containing atmosphere which causes the diffusion of Nitrogen into the metal matrix of the parts resulting in the formation of Nitrogen rich austenite. 
         [0032]    The said novel process may be optimized by performing the process in one or more steps where conditions of time between half and twelve hours, temperature between 590° C. to 720° C. and Ammonia concentration between 3 to 15%, are varied individually or severally. 
         [0033]    The vital differentiator for the said novel process is that it is possible to either achieve diffusion and consequently strength and hardness throughout the section thickness of P/M parts or to a specified depth below the part surface by controlling the said process parameters within said range. 
         [0034]    The parts initially hardened by the said Nitrogen gas alloying process in the first stage may be further subjected to an optional second stage of aging for additional hardening and strengthening of P/M parts with lower carbon content or hardening without strengthening of parts with a higher carbon content by heating the P/M parts to a temperatures between 180° C.-590° C. and holding them at temperature for a time period generally not exceeding two hours. 
         [0035]    One advantage of the said novel process is that it replaces the conventional method of strengthening and hardening P/M parts by heating the parts to above 800° C. followed by rapid cooling or quenching, frequently in oil. 
         [0036]    This novel adaptable process technology reduces part distortion (as it eliminates rapid cooling from a high temperature), fosters better process economics by reducing energy utilization (as lower processes temperatures are employed) and eliminates pollution associated with the traditional oil quenching process. 
         [0037]    Another advantage is that either one or both stages of the novel process may be performed in any type of furnace and may be performed as an independent stand-alone process or as an in-line process. 
       DESCRIPTION OF THE INVENTION 
       [0038]    The first stage of the process according to the invention consists of one or more steps of different combinations of temperature, gas composition and time performed to achieve diffusion of nitrogen into the metal matrix of P/M parts so as to primarily cause the formation of nitrogen rich austenite along with a certain amount of hard transformation products and interstitial Nitrogen, which increases the hardness and strength of the parts. An additional and subsequent second stage of aging is optionally performed on parts processed as disclosed above in the first stage, to convert this nitrogen rich austenite into hard transformation products and thereby further increase the hardness and strength of low carbon containing P/M parts and increase the hardness without a corresponding increase in strength of parts with a higher carbon content. The first stage of gas alloying can be performed in-line with the prior sintering process, in a module attached to the sintering furnace or as an independent stand alone process in a separate furnace. The second stage of aging can be similarly performed in-line with the first stage of the process or as an independent stand alone process. The process has been primarily devised to impart hardness and mechanical strength to all P/M parts including those with little or no alloying elements and without rapid cooling or quenching. 
         [0039]    According to one embodiment of the invention the first and the second stages of the said inventive process can be done as a conjunct process, in line with the prior process, sintering in case of the first stage and the first stage of the process in the case of the second stage, either in the same furnace or in a module attached to the prior furnace or in another furnace placed in line to the prior furnace. 
         [0040]    According to another embodiment of the invention the first and the second stages of the said inventive process can be done sequentially but in different furnaces. 
         [0041]    According to another embodiment of the invention either the first or the second stages can be done either in batch furnaces or in continuous furnaces 
         [0042]    In one more embodiment of the invention the process parameters in the first stage of the process can be controlled in one or more steps of varying time, temperature and atmosphere gas composition to optimize the process with respect to the characteristics of the P/M part, application of the part, logistics and process economics. 
         [0043]    In one more embodiment of the invention the process parameters in the first stage of the process such as time, temperature and atmosphere gas composition can be controlled to bring about nitrogen diffusion throughout the cross section of the P/M part to the extent allowed by the density and section thickness of the P/M part. 
         [0044]    The first stage of the novel process consists of heating to and holding at a temperature between 590° C. to 720° C. unalloyed or low alloyed P/M parts in an atmosphere containing a Nitrogen donor such as Ammonia in either batch or continuous furnaces. The concentration of ammonia during the first stage is maintained between 3% to 15%. 
         [0045]    The second stage of the inventive process is an ‘aging’ process which may be conducted either as an in-line process or as a stand-alone independent process that involves the heating of P/M parts that have fully or partially cooled after the first stage to a temperature between 180° C. and 660° C. in an atmosphere of plain air or Nitrogen or in the event the second stage is combined with yet another process such as for example, steam treatment, then in the atmosphere that such process is carried out in, to effect conversion of nitrogen rich austenite formed during the first stage of the inventive process to hard transformation products that further improves the strength and/or the hardness of the P/M parts depending on the carbon content of the parts. 
         [0046]    This said first stage can consist of one or more steps where conditions of time, temperature and gas composition, are varied individually or severally to meet the demands raised by the application the P/M part is used for, the extent of alloying elements in the part, the density and size of the part, the type of furnace employed, availability of utilities as well as economic considerations and such variations do not affect the scope of the claims as appended as they only allow dynamic use of basic principles devised for achieving utility end points as stated in the objectives. 
         [0047]    In one embodiment of the inventive process the Ammonia concentration of the process atmosphere can be varied (between 3%-15%) in different steps of the first stage of the process while the temperature is kept constant. 
         [0048]    In another embodiment of the inventive process the Ammonia concentration in the process atmosphere can be pulsed or changed at periodic intervals in different steps of the first stage of the process while the temperature is kept constant or also varied. 
         [0049]    In another embodiment of the inventive process the Ammonia concentration of the first stage of the process atmosphere can be kept constant while the temperature is varied in different steps of the process. 
         [0050]    In another embodiment of the inventive process the atmosphere gas composition the P/M part is exposed to either while being heated to or cooled from the process temperature to eliminate the presence of air can be Nitrogen or any other inert gas in case when the process is carried out without a plasma field. It is clarified that molecular Nitrogen will not react with metal, for which nascent Nitrogen which comes from cracking of Ammonia on the part surface is required. 
         [0051]    In another embodiment of the inventive process the P/M part can be processed in vacuum while being heated to the process temperature and can cooled down in vacuum or in Nitrogen or any other inert gas. 
         [0052]    In one embodiment of the inventive process the temperature can be varied from 0.5 hour after the part has reached process temperature to 12 hours depending on the process temperature employed, the part size, the part density and the depth of nitrogen diffusion below the part surface that is required. 
         [0053]    The size, density, shape retention and chemical composition of P/M parts, the process temperature, the process economics, the type and capacity of the processing furnaces, the availability of utilities and properties required in the P/M part are factors that govern the choice of parameters to be applied when performing the said inventive process. Application of specific conditions are to be decided depending on one or more of these factors and these have been clarified by way of the examples provided below which are intended only to explain the novel process technology further. However persons skilled in the art would know that such references would in no way limit the scope of the invention as appended in the claims. 
         [0054]    For example P/M parts of Iron with 2% Copper and 0.5% Carbon having a density of 6.8 grams per cubic centimeter were subjected to a two step process at a constant temperature of 675° C. in an atmosphere with an ammonia concentration of 10% for one hour during the first step followed by a second step of another hour where the atmosphere gas is entirely Nitrogen without any Ammonia, to achieve an improvement in hardness from 200 Vickers Hardness Scale in the sintered P/M parts before being subjected to the inventive process to above 320 Vickers Hardness Scale throughout the cross section of the part without the formation of iron nitrides on the surface. 
         [0055]    For another example P/M parts of Iron with 2% Copper and 0.5% Carbon having a density of 6.8 grams per cubic centimeter were subjected to a single step process at a constant temperature of 700° C. in an atmosphere with an ammonia concentration of 5% for half hour to achieve an improvement in hardness from 200 Vickers Hardness Scale in the sintered P/M parts before being subjected to the inventive process to 650 Vickers Hardness Scale on the surface gradually reducing to the original core hardness of 200 Vickers in a distance of 0.65 mm below the parts surface with the formation of metal nitrides on the surface. 
         [0056]    For another example P/M parts of Iron with 2% Copper and 0.5% Carbon having a density of 6.8 grams per cubic centimeter were subjected to a two step process at a constant temperature of 660° C. in an atmosphere with an ammonia concentration of 10% for 2.5 hours during the first step followed by a second step of another 1.5 hours in an atmosphere with an ammonia concentration of 3% to achieve an improvement in hardness from 200 Vickers Hardness Scale in the sintered P/M parts before being subjected to the inventive process to above 492 Vickers Hardness Scale on the surface gradually reducing through the cross section of the part till 236 Vickers at the core, without the formation of iron nitrides on the surface. 
         [0057]    The second stage of the inventive process, the ‘aging’ process was performed at a temperature of 350° C. for a period of 2 hours in air for all the examples described above. 
         [0058]    For yet another example three types of P/M iron parts all with 2% Copper, in the first case with 0.5% carbon and a density of 7.2 grams per cubic centimeter, in the second case with 0.8% Carbon and a density of 6.8 grams per cubic centimeter and in the third case with 0.9% Carbon and a density of 7.2 grams per cubic centimeter were subjected to a two step process at a temperature of 650° C. in the first step of half hour with an Ammonia concentration of 6% followed by a second step where the temperature was maintained at 700° C. for half hour with an Ammonia concentration of 4%. All the processed parts were aged at temperatures of 200° C., 350° C., 450° C. and 540° C. in air for a period of 1 hour in all cases. All parts were subjected to a ‘crush test’, a measure of radial crushing strength and it was seen that parts with lower carbon content (of 0.5%) the strength was significantly higher than the strength of the ‘as sintered’ part after the first stage of the process. The strength first reduced as the aging temperature increased and then increased, in all cases being higher than the ‘as sintered’ part except in one case where it was equal to the strength of the ‘as sintered’ part. Parts with a higher carbon content of 0.8% exhibited higher strength after stage one of the process compared to the ‘as sintered’ strength but lower at all aging temperatures. Parts with a higher carbon content of 0.9% exhibited higher strength in the ‘as sintered’ condition after stage one of the process compared to the any of the parts that were processed. In all cases the hardness of all parts however processed were higher than the hardness of the ‘as sintered’ parts. 
         [0059]    The present invention is of course, is in no way restricted to the specific disclosure found herein but will also include any modifications within spirit and the scope as appended in the claims. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0060]      FIGS. 1 to 4  represent time temperature graphs of different embodiments of the inventive processes. 
           [0061]      FIG. 5  shows the microstructure of the surface of a P/M part which has been subjected one embodiment of the novel process wherein hard transformation products have been formed throughout its cross section without the formation of iron nitrides 
           [0062]      FIG. 6  shows the microstructure of the core of the same P/M part mentioned above wherein hard transformation products are seen. 
           [0063]      FIG. 7  shows the microstructure of another P/M part which has been subjected to yet another embodiment of the said novel process wherein a shallow iron nitride layer is visible along with a substrate consisting predominantly of hard transformation products. 
           [0064]      FIG. 8  shows the core of the same P/M part described in  FIG. 7 , that consists predominantly of ferrite and pearlite with less hard transformation products compared to the microstructure shown in  FIG. 6 . 
           [0065]      FIG. 9  shows the hardness profile from surface to core of P/M parts that have been subjected to some embodiments of the novel process that show the increase in hardness throughout the cross section of the parts compared to the hardness of parts that have not been subjected to the process, shown as the flat line at the bottom and the hardness of parts that have been subjected to conventional heat treatment process of heating and quenching in oil, the two flat lines shown at the top of the graph. 
           [0066]      FIG. 10  shows the hardness profile from surface to core of P/M parts that have been subjected to some other embodiments of the novel process that show increase in hardness at the surface of the parts gradually reducing towards the core of the parts compared to the hardness of parts that have not been subjected to the process, shown as the flat line at the bottom and the hardness of parts that have been subjected to conventional heat treatment process of heating and quenching in oil, the two flat lines shown at the top of the graph. 
           [0067]      FIG. 11  Radial crushing strength of as sintered parts compared with satgel &amp; stage 2  conditions 
           [0068]      FIG. 12  is a photomicrograph of Gas alloyed parts (surface structure) after Stagel 
           [0069]      FIG. 13  is a photomicrograph of the same  FIG. 12  (core structure) after Stagel 
           [0070]      FIG. 14  is a photomicrograph of the same  FIG. 12  (surface structure) after Stage 2   
           [0071]      FIG. 15  is a photomicrograph of the same  FIG. 12  (core structure) after Stage 2   
           [0072]      FIG. 16  is a photograph of a scanning electron microscope shown alongside an energy dispersive X-ray of the photograph which shows the Nitrogen concentration on the surface of a P/M part subjected to one embodiment of the novel process. 
           [0073]      FIG. 17  is another photograph of a scanning electron microscope shown alongside an energy dispersive X-ray of the photograph which shows the Nitrogen concentration on the surface of the P/M part subjected to another embodiment of the novel process. 
           [0074]      FIG. 18  is a graph of the Nitrogen concentration from the surface to core of the P/M parts described in  FIGS. 16 &amp; 17 . 
       
    
    
     BEST METHOD OF WORKING THE INVENTION 
       [0075]    The said novel process technology has been devised for improving the hardness and other mechanical properties of iron and steel Powder Metallurgy (P/M) parts in which the first stage is the alloying the parts with Nitrogen gas which causes the formation of an austenitic phase in the metal matrix of the parts throughout the section thickness or to a controlled depth beneath the surface of the parts, in addition to the formation of hard transformation products and interstitial Nitrogen. This is followed in some cases by a second stage of “aging” which causes an additional improvement in hardness in all P/M parts thus processed as well as strength in P/M parts with less carbon content, by the conversion of the bulk of the above mentioned austenite phase to hard transformation products. The first stage of the process can be performed in one or more steps where time, temperature and atmosphere composition is varied depending on the size, density and chemical composition of the P/M part and the use the parts are put to. The various permutations and combinations may be easily understood by reading the varying embodiments of the invention described above. Examples have been suggested to illustrate the various embodiments described that may be practiced to achieve the desired utility and advantages provided by the inventive process.