Hardness refers to the ability of a material to resist the pressure of hard objects into its surface, is a measure of the degree of hardness and softness of the material performance indicators. Hardness to a certain extent reflects the strength, toughness and elasticity of metal materials and a series of mechanical properties, in the petroleum and chemical industry is one of the most common metal material performance testing indicators. Common metal material hardness test methods are mainly Brinell hardness test, Vickers hardness test and Rockwell hardness test.
Brinell hardness uses a certain diameter D carbide ball, pressed into the surface of the test piece under the action of the specified test force P. After holding the load for a period of time and unloading the test force, the surface of the test piece is left with an indentation, and the average pressure on the unit surface area of the indentation is used as the Brinell hardness test value. Because the diameter of the spherical indenter used in Brinell hardness test is larger, the indentation area obtained is larger, thus the hardness test value is slightly affected by the microscopic segregation of the specimen and the uneven composition, and it has the characteristics of stable test data, high measurement accuracy and good repeatability, which is widely used in the petroleum and chemical industry.
Shanghai HY Industry Co.,Ltd researchers in the different welding process Inconel 625 nickel-based alloy pipe joint weld Brinell hardness test, according to GB/T 231.1-2018 “metal materials Brinell hardness test Part 1: test method” standard recommended test force and indenter diameter Square ratio and as far as possible to choose the large diameter indenter two principles, the choice of HBW 10/3000 (10 indicates the diameter of the steel ball indenter 10mm, 3000 indicates that the test force of 3000kg) measurement scale for testing, found that the indentation shape presents irregular oval, indentation boundary bumpy, not pressed area of the metal surface showing the phenomenon of extrusion bulge, which seriously affects the test results This seriously affects the accuracy of the test results. In response to the above-mentioned problems, the researchers through the hardness test under different measuring scale conditions, comparative analysis of the impact of the measuring scale on the Brinell hardness measurement value of nickel-based alloy pipe joints, to determine the applicable welding process Inconel 625 nickel-based alloy pipe joints weld Brinell hardness measurement scale.
1.1 Test materials and test equipment
Test material Inconel 625 nickel-based alloy, the use of argon arc welding priming, manual arc welding filler welding of nickel-based alloy tube welded joints (handle welding) recorded as No. 1 specimen, the use of argon arc welding of nickel-based alloy tube welded joints (argon arc welding) recorded as No. 2 specimen, the use of argon arc welding stacking process for large diameter carbon steel + nickel-based alloy composite tube welded joints (stacking) recorded as No. 3 Specimen. The chemical composition of the weld metal of different specimens conforms to AWS A5.14/A5.14M – 2018 Specification for Nickel and Nickel-alloy Bare Welding Electrodes and Rods standard for alloy 625 Nickel-Based Alloy Pipe Technical requirements. The test equipment is a digital display Brinell hardness tester with electronic automatic loading and automatic measurement. The hardness is measured manually due to the irregular indentation shape that cannot be identified by the automatic measurement method.
1.2 Test method
According to GB/T 231.1-2018, the standard recommended ratio of test force to indenter diameter squared 30 was selected, corresponding to the Brinell hardness measurement scale of HBW 10/3 000, HBW 5/750 and HBW 2.5/187.5. Brinell hardness tests were conducted on specimens No. 1, No. 2 and No. 3, and each specimen was subjected to ten Brinell The indentation diameter was measured from four different directions with a holding time of 15s and a test temperature of 22°C.
2 Test results and discussion
2.1 Brinell hardness indentation morphology
The surface indentation shape of the weld area of the specimen are irregular oval; with the reduction of the diameter of the steel ball indenter, the surface indentation shape of the weld area of specimen No. 1 and No. 2 gradually tends to round; under different measuring scale conditions, the surface indentation of the weld area of specimen No. 3 is regular round, and the indentation edge is smooth and smooth.
Nickel-based alloy hardness is high, its local resistance to deformation. Hardness reflects the ability of the material internal grains and grain boundaries and other tissues to resist breakage and deformation when pressed into the load, due to the existence of microstructure heterogeneity in the weld area, the ability of the tissue at the edge of the indentation to resist deformation varies.
2.2 Brinell hardness test results
In the HBW 5/750 measurement scale conditions, the measured hardness of the weld area of the specimen extreme difference are less than the other measurement scale conditions, indicating that in the measurement scale conditions, the measured hardness of the weld area of specimen No. 1 and No. 2 is more uniform. Under the HBW 10/3000 measuring scale, the extreme difference of hardness in the weld area of the specimen is slightly smaller than under the other measuring scale conditions, indicating that the hardness of the weld area of specimen No. 3 is more uniform under the measuring scale conditions.
The Brinell hardness in the weld area of specimen No. 1 is the largest, followed by specimen No. 2, and the smallest in specimen No. 3; the difference in Brinell hardness in the weld area of the same specimen measured under different measuring scale conditions is basically within 10, indicating that the ratio k of the same load to the square of the indenter diameter corresponds to the Brinell hardness measured under different measuring scale conditions is basically the same. According to the similarity law of Brinell hardness, the Brinell hardness measured under different measuring scales is the same and comparable under the condition that the press-in angle is kept constant (i.e., the k value is kept constant). It can be seen that the greater the hardness of the weld area of the specimen under the same measuring scale conditions, the more pronounced the irregularity of the indentation shape.
Hardness of nickel-based alloys is related to the type, morphology and distribution of precipitation phases. The microstructure of the weld area of the three specimens is mainly austenite, which shows dendrite morphology, and the direction of dendrite growth is the same, and a large number of γ″ precipitation strengthening phases are precipitated at the grain boundaries.
Nickel-based alloy metal mobility is poor, too high heat input will lead to a slow cooling rate of the melt pool, high temperature residence time is too long will make the grain coarsening. 1 specimen welding process heat input of 0.9 to 1.0kJ/mm, No. 2 and No. 3 specimen welding process heat input are 1.4 to 1.6kJ/mm. 2 and No. 3 specimen welding process is the same, due to the thermal conductivity of the base metal of No. 2 specimen Large, effective heat transfer during the welding process, its high-temperature residence time is shorter than the No. 3 specimen. According to the heat input and high-temperature residence time is different, the grain size of the three specimen weld area from large to small in order for the No. 3 specimen, No. 2 specimen, No. 1 specimen, the larger the grain size of austenitic tissue, the smaller the Brinell hardness.
Under different measuring scale conditions, the Brinell hardness indentation edges of the weld area of the nickel-based alloy tube of the overlay process are smoother and smoother. In the HBW 5/750 and HBW 2.5/187.5 measurement scale conditions, the shape of the Brinell hardness indentation tends to be round in the weld area of nickel-based alloy tubes using the hand-held welding and argon arc welding processes. The same load and indenter diameter squared ratio k conditions using different measuring scales to measure the Brinell hardness is basically the same.
For overlay welding process of nickel-based alloy pipe joints, it is recommended to use the HBW 10/300 measuring scale. For handle welding and argon arc welding process of nickel-based alloy fittings, HBW 5/750 measuring scale is recommended.
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