Casting is an important branch of the machinery industry. The presence of graphite gives cast iron excellent casting properties, cutting performance, wear reduction, vibration reduction and low notch sensitivity. Moreover, the casting production equipment is simple and the manufacturing cost is low, so it is widely used in industrial production. In all kinds of machinery products, according to the quality calculation, the proportion of castings accounted for more than 50%.
The first quality index of castings is the mechanical properties, there are two main methods to test the mechanical properties of the workpiece, one is the tensile test, and the other is the hardness test. Tensile test tests the tensile strength, yield strength and elongation of the workpiece, while the hardness test reflects the comprehensive performance of a variety of physical quantities such as elasticity, plasticity, strength, toughness and wear resistance of the material under the respective specified conditions. Almost every product in the U.S. casting standards provide for tensile testing. Most products specify the hardness test.
Tensile test equipment is complex, high investment, requires professional staff, requires preparation of specimens, low test efficiency and high cost. Hardness test equipment is simple, easy to master, indentation is small, can be regarded as nondestructive testing, and can directly test finished or semi-finished workpieces. Testing efficiency is high, and can be used for piece-by-piece testing of batches of workpieces. With the progress of hardness tester manufacturing technology, a variety of portable instruments, especially high-precision portable instruments continue to appear, making the hardness test to achieve simple, fast and accurate. It makes the hardness testing on site, hardness control on production line and accurate hardness testing of large workpieces possible.
Hardness testing and tensile testing are basically testing the ability of metals to resist plastic deformation. To some extent, the two tests are testing similar properties of metals. The test results of both tests are fully comparable with each other, and for most metallic materials, the hardness and tensile strength values can be converted to each other by looking up the table. Therefore, the hardness test is increasingly being used for testing the mechanical properties of materials, and the tensile test is less frequently used.
The following is the American standard ASTM on the hardness requirements of castings.
First, the American standard ASTM on the hardness of castings requirements
Gray iron castings (ASTM A48-92)
Applicable to the main consideration of tensile strength of the general engineering gray iron castings, castings according to the tensile strength of different casting test bars graded. In such cast iron parts, the chemical composition is of secondary importance relative to the tensile strength. Castings in the order or production, according to the performance of individual casting specimens into a number of grades, each grade using a number followed by a letter, the number indicates the minimum tensile strength of the individual casting test bar, the letter indicates the specifications of the test bar. For example: gray iron castings, ASTM A48, 30B grade that the standard ASTM A48 production, the minimum tensile strength of 30 thousand pounds / inch 2 (207MPa), the nominal diameter of the test bar is 1.2 inches (30.5mm). The standard states that "in the production plant and the purchaser to reach a written agreement, the casting is required to meet the hardness, chemical composition, microstructure, pressure leakage, X-ray inspection without defects, dimensions, surface accuracy and other requirements are necessary.
Gray iron castings for motor vehicles (ASTM A159-88)
Applicable to sand mold casting, gray iron castings used in automobiles, tractors and related industries. Hardness requirements.
The foundry shall adopt the necessary control and inspection techniques to ensure that the casting conforms to the specified hardness range. Brinell hardness shall be tested according to the ASTME10 test method, after sufficient thickness of material has been removed from the surface of the casting to ensure the representativeness of the hardness readings. Unless otherwise agreed, a 10 mm steel ball and a 3000 kg load shall be used. The area on the casting to be tested for hardness and its location shall be agreed between the supplier and the customer and indicated on the drawing.
Ductile Iron Castings (ASTM A536-84)
Applicable to castings made of ductile iron, which contains spheroidal graphite and essentially no other form of graphite. Casting grades are expressed in terms of "tensile strength - yield strength - elongation", for example: Grade: 80-55-06 represents tensile strength 80,000 lb/in2 (552MPa), yield strength 55,000 lb/in2 (379MPa), and elongation. 379MPa), elongation of 6.0% (2 inches or 50mm). Castings shall be subjected to appropriate heat treatments such as annealing, normalizing, quenching and tempering, etc. When specified in the contract or order form, the casting shall meet the requirements of hardness, chemical composition, microstructure ......, etc.
Austenitic ductile iron castings (ASTM A439-89)
Applicable to austenitic ductile iron parts mainly used for heat resistance, corrosion resistance and wear resistance. The castings should be heat treated such as stress relieving, stability treatment or annealing. Isothermal Quenched Ductile Iron Castings (ASTM A897M-90) For ductile iron castings requiring isothermal quenching heat treatment. Isothermal quenching can make the same casting of different parts or the same furnace iron cast into different castings between the mechanical properties of the difference is reduced. Application of isothermal quenching heat treatment can expand the range of properties that can be obtained on ductile iron parts.
Pearlescent malleable castings (ASTM A220 M-88)
For pearlite malleable castings for general engineering use working from room temperature to 400°C. If the purchase contract requires a hardness test, the acceptable range of hardness should be stated and the test area should be clearly shown on the accompanying drawings. Wherever possible, the Brinell hardness test method specified in ASTM E10 should be used. And test conditions of 3000 kg/10 mm ball should be used as far as possible, or 1500 kg/10 mm ball if the size or shape of the workpiece does not allow it. In the special case where the Brinell hardness tester cannot be used, the Rockwell hardness test method can be used instead in accordance with ASTM E18.
Malleable cast iron castings for automobiles (ASTM A602-87)
Applicable to the automotive industry and the same type of industrial products used in ferritic, pearlite, tempered pearlite and tempered martensitic grade malleable cast iron castings. Castings should be heat treated. Hardness requirements castings should implement the necessary control and inspection processes to ensure compliance with the specified hardness range. Hardness readings shall be taken in accordance with ASTM E10, after removal of sufficient thickness of material from the surface of the casting to ensure the representativeness of the hardness value. The surface or area of the casting to be checked for hardness shall be determined by mutual agreement between the supplier and the customer and as shown on the drawings.
Wear-resistant cast iron (ASTM A532/A532M-87)
This standard applies to a group of alloyed white cast irons to ensure high wear resistance for applications in the mining, mineral processing, clay handling, and manufacturing industries. Grade II and III alloys are often ordered in a heat-treated condition with a maximum hardness of 400 HB. hardness testing can be performed on any part of the original surface of the casting. Indentations can be made on the original surface of the casting or 1/8" deep into the original surface. Hardness is tested in accordance with the methods specified in the following ASTM standards. The preferred method is the ASTM E10 Brinell test method using a tungsten carbide ball and 3000 kg force. As an optional method, the Rockwell method of ASTM E18 standard with a Rockwell C scale, diamond cone indenter and 150 kg force can be used.
Second, the application of hardness tester on casting parts
1. Brinell hardness tester
Casting hardness test preferred Brinell hardness tester, especially the coarse grain of gray iron castings, can only use Brinell hardness tester, and try to use 3000kg load, 10mm ball test conditions, when the casting size is small, can also choose Rockwell hardness tester.
In the casting parts hardness testing priority to use Brinell hardness tester because of the following two points.
a, cast iron parts are usually not uniform organization, larger grain, containing carbon, silicon and other impurities than steel, in different tiny areas or different points on the size of the hardness will vary. The indenter size of the Brinell hardness tester is larger, the indentation area is larger, can measure the average value of the material hardness within a certain range, so the use of Brinell hardness tester test accuracy is higher, the dispersion of the hardness value is smaller, the measured hardness value is more representative of the actual condition of the workpiece hardness. So Brinell hardness tester is widely used in the foundry industry.
b. Tensile strength is the first mechanical property index of castings, and almost all the standards of casting parts have requirements on tensile strength.
And the Brinell hardness value and tensile strength value of castings have a very close relationship, and the two values can be converted to each other. The tensile strength of gray cast iron can be calculated by the following formula.
σb = 1.82 (HB) 1.85
The relationship between the two can also be obtained by looking up the table. The tensile strength of the workpiece can be obtained quickly and easily by testing the Brinell hardness value. Thus improve the efficiency of testing, reduce the cost of testing.
2. Rockwell hardness tester
Rockwell hardness tester is also commonly used in the hardness test of cast iron. Where the grain is fine workpiece, if there is not enough area for Brinell hardness test, can also be Rockwell hardness test, for pearl malleable cast iron, chilled cast iron and cast steel parts, you can use HRB or HRC scale, if the material is not uniform, should be measured several readings, take its average value. Rockwell hardness tester test fast, convenient, small indentation, can directly test the finished workpiece, suitable for batch production of finished or semi-finished workpiece piece by piece testing.
3. Shaw hardness tester
In individual cases, some type body larger casting parts, do not allow the cutting of specimens, but also can not be additionally cast for hardness testing test block, then hardness testing will encounter difficulties. For such cases, the common approach is to test the hardness of the casting with a portable Shore hardness tester on a clean surface after finishing. For example, the standard for rolls, which is widely used in the metallurgical industry, requires the use of a Shore hardness tester for hardness testing. Shore hardness tester due to the use of dynamic hardness testing principle, affect the hardness test results of more factors, test accuracy is much lower than the use of static indentation hardness testing principle of Brinell hardness tester and Rockwell hardness tester. For this reason, a hardness comparison roll is also recommended in the roll standard. The hardness comparison roll plays the role of a specimen whose hardness value is dependent on the method of cutting the specimen to get an accurate test. After using the Shore hardness tester to test the rolls, check the accuracy of the Shore hardness tester on the comparison rolls.
4. Richter hardness tester
At present, the Richter hardness tester is widely used in the hardness testing of casting parts. Richter hardness tester is an improvement of the Shore hardness tester. It is also the use of dynamic hardness test principle, the use of computer technology to achieve the miniaturization of the hardness tester, electronic, easy to use, the test results can be easily converted to Brinell hardness value, and thus widely welcomed. However, like Shore hardness tester, the accuracy of the Richter hardness tester is not high, affecting the test accuracy of more factors, requiring the workpiece surface with a high degree of finish, and the lack of authoritative hardness conversion table, hardness conversion will also bring a large error. Therefore, the measurement results of the Richter hardness tester are often used by people as reference values in China, and the Richter hardness tester is mainly used for occasions with a wide range of hardness requirements for workpieces. In addition, the Richter hardness tester as an informal hardness testing method is not widely recognized in the International Organization for Standardization, and is not adopted in the product standards of foreign casting parts. In international trade, the measurement results of the Richter hardness tester will not be accepted by most foreign investors. Many casting parts are medium to large workpieces, some weighing several tons, and cannot be moved to the benchtop hardness tester for testing. Accurate hardness testing of castings is mainly done with individually cast test bars or test blocks attached to the casting. However, neither the test bar nor the test block can completely replace the workpiece itself, even if it is the same furnace iron, the casting process and heat treatment conditions are the same, because of the huge difference in size, which will cause the heating rate and especially the cooling rate of the two to be different, it is difficult to make the two have exactly the same hardness, due to this reason many customers are more concerned and believe in the hardness of the workpiece itself. This requires a portable and accurate hardness tester to test the hardness of the casting
