Core tip: The MIM Association uses the same grade system as AISI-SAE when developing technical specifications for MIM materials. These designations were chosen because MIM parts are often used to replace products of the corresponding wrought material already in use. When indicating that a material is manufactured by the MIM process, "MIM" shall be added before the material. For example, 316L stainless steel manufactured by the MIM process may be referred to as "MIM-316L".
American MPIF Standard 35-Material Standard for Metal Injection Molding Parts
Notes and definitions of material standards for MIM parts
(1) Naming of MIM materials
The MIM Association uses the same grade system as AISI-SAE when developing specifications for MIM materials. These designations were chosen because MIM parts are often used to replace products of the corresponding wrought material already in use. When indicating that a material is manufactured by the MIM process, "MIM" shall be added before the material. For example, 316L stainless steel manufactured by the MIM process may be referred to as "MIM-316L".
Before selecting a specific material, a careful analysis of the design of the part and its end use is required, including dimensional tolerances, part design, and mold design. In addition, the MIM part manufacturer and the purchaser must agree on the final performance requirements for the finished part. Issues such as static and dynamic loads wear resistance, mach inability, and corrosion resistance may also be specified.
(2) Some basic concepts and definitions
Minimum Value Concept The MPI has adopted a minimum mechanical property value concept for powder metallurgy materials used in structural parts. When manufacturing parts using the MIM process, these values can be used as a basis for the user to select a material for a specific application. In addition to the minimum mechanical property values, standard values for other properties are listed to assist the user in selecting a material. Thereby allowing the user to select and determine the appropriate MIM material and the most appropriate properties for a particular application. The data provided specify the minimum mechanical property values of the material and list the standard mechanical property values that can be achieved under industrial production conditions. The mechanical properties can be enhanced and other service properties can be improved through a more complex process.To select the best material available in terms of both performance and price, it is most important for the user to discuss the purpose of the part with the MIM part manufacturer.
Minimum Values The minimum values of MIM materials are expressed in terms of yield strength (0.2% residual deformation method), ultimate tensile strength, and elongation for all materials in the as-sintered and/or heat-treated States. Because the density of the MIM material is close to the true density, its performance is similar to that of the wrought material.
For the purposes of this standard, the tensile properties used were determined from tensile test specimens specially prepared for the evaluation of MIM materials (see MPIF Standard 50 for details on MIM material test specimens). Tensile properties measured on specimens machined from mass-produced parts or on non-standard MIM specimens may differ from those measured on specimens prepared in accordance with MPIF Standard 50.
In the preparation of a specification for MIM materials, the actual method of indicating the minimum strength value is by the manufacturer
Perform static or dynamic acceptance tests with the user using the first parts produced and a mutually agreed upon method of applying force to the parts. For example, depending on the design of a given part, it is agreed that the failure load must be greater than a given force. If the specified value is exceeded in the acceptance test, it indicates that the minimum strength value has been reached. The first batch of parts can also be used for in-service testing to show that they are qualified. Static and dynamic breaking loads are measured separately, and these data are statistically analyzed to determine the minimum breaking force for future series production parts. Parts produced in future batches that exceed this minimum force indicate that the strength specified in the specification has been achieved. Tensile specimens may also be used to qualify the strength.
These specimens and parts shall be manufactured from the same batch of material and of the same material density as the parts, and shall be sintered and heat treated together with the parts produced. However, defects that develop during the formation of the actual part may limit the properties measured with the tensile specimen. In the event that acceptance testing is not used, supplementary quality inspection of the part, such as X-ray analysis, may be required in order to achieve compliance with the minimum performance requirements.
The use of MPIF Standard 35 to establish the MIM material specification means that the material will have the minimum performance values specified in the standard unless otherwise agreed between the purchaser and the manufacturer. Of course, if a test specimen is used to determine this value, it should have the shape and other characteristics determined by the manufacturer and specially prepared for the evaluation of this material under the same conditions as the production of the part.
Standard Values For each MIM material listed; there are standard values for a set of properties (i.e., density, hardness, elongation, etc.) Some or all of which may be important for a particular application. The standard values for the densities listed are determined by interpolation from the average mechanical properties-density curves. The data of mechanical properties were derived from the "cyclic" sintering and heat treatment of the specimens.
Standard values are listed for general guidance only and should not be considered minimum values. When used in a general manufacturing process, there may be slight variations depending on the part test area selected or the specific manufacturing process used. For each material required by the purchaser, the properties listed under the "Standard Value Column" must be fully discussed with the MIM part manufacturer before the technical conditions are established. For each MIM part, the required performance values shall be specified separately for its intended use, except for the performance expressed as a minimum.
Chemical composition the chemical composition of each material lists the minimum and maximum contents of the major elements. "Other elements" are calculated by subtraction, including all other elements (reported at maximum content). These elements may include some trace elements added for special purposes.
Mechanical properties the mechanical properties data indicate minimum and standard property values that are expected to be achieved if the density and chemical composition listed for the specimen meet the standards. Of course, the mechanical properties used in this standard. Are to be determined by special specimens specially manufactured for material evaluation and special specimens sintered under industrial production conditions. Hardness values for heat treated specimens are given first for apparent hardness and then, where possible, for equivalent particle or matrix hardness values. Porosity remaining in the MIM part can have an effect on the apparent hardness reading. Matrix hardness values expressed as HRc were converted from Knoop microhardness measurements at a load of 100 GF (0.981 N).
Heat treatment Except for austenitic stainless steel, MIM materials can be heat treated to increase strength, hardness and wear resistance. MIM iron-based parts with combined carbon content of 0.3% or more can be quenching hardened and tempered. The percentages of carbon, alloying elements, and residual porosity determine the degree of harden ability at any given quench condition. The hardness can be increased to 55 HRc (650 HK) or more by using quenching. Quenching is followed by tempering or stress relieving for optimum strength and wear resistance. Tempering temperature is an important factor in determining the final hardness. When MIM iron-based parts are manufactured with no or low carbon content, surface carburizing-quenching can be performed to improve surface hardness and maintain core toughness. Martens tic and precipitation hardening stainless steels may also be heat treated to increase hardness and strength.
Gas atmosphere or vacuum treatment is recommended for heat treatment and/or carburization of MIM ferrous parts. To ensure the specified carbon content, the heat treatment process must be well controlled. Most MIM materials are well adapted to the heat treatment process using conventional wrought materials. It is recommended that the heat treatment process for any MIM material be developed in conjunction with the MIM part manufacturer in order to achieve the desired balance of the final properties of the part.
Surface Roughness The total roughness and surface reflectivity of a MIM material depends on the density, mold condition, particle size, and subsequent processing. Because the surface state of MIM material is different from that of forged material machined by cutting or grinding, the surface roughness profile given by conventional profilometer reading is incorrect.
Microstructure Microstructural analysis of MIM parts is a diagnostic tool that reveals the extent of sintering and other metallurgical information critical to the MIM process. Because of the high density achieved by the MIM process, the microstructure of the MIM material is similar to that of the corresponding wrought material. Several tests are common to most sintered MIM materials and are briefly described as follows:
The porosity of the MIM material is generally not greater than 5%. The selection of a particular cross section of a MIM part is only important when considering the formation of defects. The rough and fine polishing shall be carried out until the residual pores are exposed. The area percentage of porosity means the density of the part.
Sintered parts in the uncorroded state are often analyzed first. When the sintering is normal, the original grain boundary cannot be seen under "200 ×". The strength, plasticity and impact strength of the material are relatively high when the pores are uniformly distributed, fine and properly round.
For MIM steel, the approximate carbon content can be judged by the area percentage ratio of pearlite. Less pearlite means a lower proportion of carbon. The alloying additive of elemental nickel should be fully diffused, and the nickel-rich zone should not be misjudged as a ferrite zone. Surface decarburization should generally be avoided because of its low hardness and poor wear resistance.If the carbon content of the part is between 0.6% and 0.9%, it is obvious decarburization when the carbon content of the surface layer is less than 0.6%. A small amount of surface decarburization is not a big problem, but if the depth of the decarburization layer is greater than 0.254mm, it may be necessary to verify whether there is any damage to the function.
MIM low alloy steels are generally all martensitic in the heat-treated state. In hardened parts, the presence of network carbides will cause martensite embrittlement, so this situation should generally be avoided. A small amount of carbide at 0.127mm of the outer surface of the part is generally allowed, as well as a small amount of retained austenite and martensite.Because retained austenite transforms to brittle martensite in service, high percentages are generally avoided.
The microstructure of MIM stainless steel is generally the same as that of the corresponding wrought material, and the well-dispersed and well-rounded pores indicate normal sintering. Oxides, nitrides, or carbides in the grain boundaries may reduce their performance.
The following etchants and methods are recommended for the preparation of MIM specimens for microstructure analysis. Iron-based parts containing carbon are usually corroded in 2% nitric acid and ethanol corrosive solution. Austenitic stainless steel and precipitation-hardening stainless steel can be corroded by scrubbing in glyceregia (10ml HNO3, 20ml HCl, 30ml glycerol) corrosive solution for 1 ~ 2 min. This corrosive solution shall be discarded after 30 min.
1. Inspection method
(1) Acceptance test
It is strongly recommended that acceptance and/or failure test methods be developedby the user in conjunction with the MIM part manufacturer to ensure that the actual part conforms to the design intent. Where possible, the MIM part should be linked toactual applications, such as failure loads, bending tests, tensile tests, etc.The acceptance test data must be determined by actual tests. It is recommended that such tests be added to the material specifications specified on the drawing.
The porosity of MIM materials is generally not more than 5%, therefore, the impregnation method is not applicable. The methods generally used are as follows:
Where, D — density, G/cm3;
A — Mass of the sample in the air, G;
C — Mass of sample in water, G;
E — mass of hanging wire or basket in water, G;
ρw — density of water at test temperature, G/cm3.
Note: ① Mass A, C and E shall be measured to 1mg;
② 0.1% wetting agent must be added to the water to minimize the surface tension effect of the water when weighing the sample;
③ The temperature and density of water during the measurement are shown in Table 1. The density may also be determined with a gas pycnometer if agreed upon by the Buyer and the Seller.
Table 1 Relationship between temperature and density of water
Note: Values of ρw listed are taken from Metrolopical Handbook 145, Quality Assurance for Measurements
1990, NIST, P.9.10 and are expressed at 1 ATM in air (1 ATM = 101325 Pa)
(3) Ultimate tensile strength, yield strength and elongation
Ultimate tensile strength, yield strength and elongation are determined in the same way as for conventional iron and steel materials.
(4) Apparent Hardness
The hardness value of a MIM part, when measured by a general indentation durometer, is called the apparent hardness, which represents the combined value of the matrix hardness and the residual porosity efficiency. For MIM parts, the residual porosity has little effect on the hardness value, and the apparent hardness measures the indentation resistance. When measuring the apparent hardness of MIM materials, attention should be paid to:
The hardness, determination method, and hardness scale shall be agreed upon by the manufacturer and the purchaser for each part tested.
(5) Matrix hardness (microhardness)
Matrix hardness was measured with a microhardness tester using Knoop (KHN) or Diamond Pyramid Hardness (DPH) indenters. By minimizing the effect of porosity, the actual hardness of the structure can be determined. In order to compare with other materials, the measured value of microhardness can be converted into equivalent Rockwell hardness value.
When converting Knoop hardness to HRc hardness, it should be noted that typical conversion charts are based on a load of 500 GF (4.9 N), while for MIM materials, a load of 100 GF (0.98 N) is recommended. Since heat treated material forms martensite, fine pearlite, and bainite regions, the phases tested must be reported. It is important that the specimen be polished to expose all porosity. When the indenter is pressed into a hidden pore, the edge of the indenter impression will be curved, and this reading must be discarded.
III. MIM Material Technical Standard
(1) Low alloy steel
Include MIM materials made from prealloyed powders and mixed powders of iron powders and other alloying element powders such as Ni, Al, and C. In order to obtain various properties, the proportion of each element added and the heat treatment conditions may be different. The alloy can be hardened to obtain high strength and proper toughness. Alloys with lower carbon content may be surface carburized and quenched for wear resistance on the surface and toughness in the core.
The material properties are generally such that the alloying elements are sufficiently diffused during sintering. One with uniform structure has excellent strength performance. These materials also have good toughness due to the high density that can be achieved with the MIM process.
Applications Low alloy steels are generally used for light structural parts, especially when carburized and quenched, where high strength and hardness are required.
Microstructural residual porosity should be small, uniformly distributed, and relatively rounded. The microstructure of the sintered body should contain different amounts of ferrite and eutectoid depending on the carbon content.
The nominal chemical composition is shown in Table 2. The properties of low alloy steel MIM material and the physical-mechanical properties of MIM low alloy steel are shown in Table 3.
Table 2 Nominal chemical composition (mass fraction) of MIM low alloy steel
Table 3 Physical-Mechanical Properties of MIM Low Alloy Steel (MPIF Standard 35. (Edition 1993-1994)
(2) Stainless steel
Include MIM materials made from prealloyed separately or elemental powder formulated stainless steels including grades of austenitic stainless steels, duplex stainless steels, and precipitation hardened stainless steels.
Material properties the strength, toughness, and corrosion resistance of these materials are improved by the high density that can be achieved with the MIM process.
Applications there are several grades of MIM stainless steel, each with special properties and a wide range of applications.
The parts made of the material have good comprehensive strength and toughness.
Composite Tissue. Compared with 316L, its corrosion resistance is similar, but its apparent hardness is higher, and its mechanical properties are more or less improved. These alloys are ferromagnetic.
Grade of stainless steel. Because of its low carbon content, its corrosion resistance is generally better than that of 400 series stainless steel.
Different properties and hardness can be obtained by changing the aging temperature.
The microstructure of the MIM material shall be the same as that of the forged material, except that it shall have uniformly dispersed and properly rounded pores, and there shall be no traces of original grain boundaries. Internal oxides, nitrides, and chromium carbides are detrimental to performance.
Chemical composition the nominal chemical composition is shown in Table 4.
Table 4 Chemical composition (mass fraction) of MIM stainless steel
Properties of MIM stainless steel the physical-mechanical properties of MIM stainless steel are shown in Table 5.
Table 5 Physical-Mechanical Properties of MIM Stainless Steel (MPIF Standard 35. (Edition 1993-1994)
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