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The Non-Destructive Testing of the Quality of Hard Alloy Products Yu Fengnian The Heilongjiang North Tools Factory Mudanjiang-157000 CHINA Contact

Abstract

This paper will introduce the method of non- destructive testing to the quality of hard alloy products and the research results. The non-destructive test to hard alloy is use mainly the ultrasonic and magnetic method: using ultrasonic method to detect the flaw of products in order to remove the flawed;measuring the sound speed,bottom echo and coercivity to evaluate grain size and using properties of products;then evaluating the quality by comprehensive results of ultrasonic testing and coercivity.

Hard alloy is a powder metallurgy material,which is made by pressing metal powder to shape and then sintering in the high temperature.

Because of raw material,the process (mixing,shaping, sintering,heat treatment etc.) and soon hard alloy products have defects,different grain size and different mechanical property.This kind of products stand very high pressure when using.If its mechanical property dose not reach the standard,its using time will be reduced,or even it break to uselessness when it is used only once.In order to pledge the quality of products,we must conduct a hundred per cent test to them,which is to finished by only non-destructive testing. We have made research of non-destructive testing method of the hard alloy products,

using mainly ultrasonic method and magnetic method:removing the flawed by detecting the flaws of products;evaluating grain size and properties of products by measuring sound speed,ultrasound attenuating and coercivity,and then evaluating the quality of products by the comprehensive results of ultrasound testing and measuring coercivity.This non-destructive testing,which conducts the complete test to products,pledges the quality of products.

The sound speed measurement of products

It is well known that the sound speed of the longitudinal wave of the solid materials

CL =

(1)

C_L is the longitudinal wave sound speed of the materials;E is the elastic modulus of the materials;r is the density of the materials,and s is Poisoon ratio. The sound speed of the materials is related to elastic modulus of the materials, so it also related to the strength of the materials. Therefore, measuring the sound speed can estimate the strength of the materials in a certain field.

Measure the time t, which is spent from the mth bottom echo of products to the nth, by the thickness of products s and t, it is that the longitudinal wave sound speed is

CL=2(n-m)s/t

(2)

The measurement of attenuation

By the theory that ultrasound transmit in the medium, it is know that the energy attenuation of ultrasound wave is three elements:spread attenuation, scattering attenuation and absorption. The spread attenuation is resulted while ultrasound is spreading in medium; the scatting attenuation is produced while ultrasound is scattering on the interface of medium crystal grain; the absorption attenuation is caused while medium absorbs the ultrasound energy to heat. The attenuation coefficient expresses for attenuation degree of ultrasound caused by some material. The total attenuation coefficient of material is

a= ad + asc + aa

(3)

In this formula, a _d is spread attenuation coefficient; a _sc is scattering attenuation coefficient; a _a is absorption attenuation coefficient. In the course of measurement,the ultrasonic field produced by the same probe in products which the same length and shape may be regarded as the similar, therefore, the scattering attenuation may be regarded the same. (The absorption attenuation of solid materials is very small )In the formula of the scattering attenuation coefficient of materials

asc = C2Fd3f4        (d«l)

(3)

C₂ is scattering coefficient, F is anisotropic coefficient, d is grain diameter, f is ultrasonic frequency, l is ultrasonic wave length. ( Among hard alloy products,5MHz ultrasonic longitudinal wave is approximately 1.35mm, grain diameter is largest 2 m m, which is further smaller than wave length) In the course of measuring attenuation, ultrasonic frequency is the same. The scattering coefficient C₂ and anisotropic coefficient F in every products may be regarded similar, therefor, scattering attenuation is determined by grain diameter d. From above,measuring ultrasonic attenuation can determine the grain degree, and ultrasonic attenuation may be determined by measuring the bottom echo highness of products. The bigger attenuation is, the lower bottom echo is (gain is bigger);the smaller attenuation is, the higher bottom echo is(gain is smaller). For example,there are two products with lower bottom echo respectively 43dB and 42dB,their grain degrees are bigger both between 1.6-2.0m m; and the other two products with higher bottom echo,is 34dB, their grain degrees are smaller, between 0.8- 1.0m m. Using immersion testing to measure bottom echo can assure the same incidence condition of every products.

The relationship between non-destructive testing results and products quality

To conduct ultrasonic flaw detection, and to measure sound speed,bottom echo and coercivity, and then to evaluate the quality of products by the measuring and testing consequences. Table 1 shows some testing results,mechanical property,dissection results and using condition, of some typical products.From the Table 1, product 1,2 have lower sound speed, lower bottom echo,bigger grain size, lower coercivity, and its strength is lower, these products were crushed when using only once. The products with higher sound speed, higher bottom echo,higher coercivity and no flaws generally have thinner grains, higher strength and longer service time, which can be see from products 4 and 5. Also seen from Table 1, bottom echo highness and grain size have correspondent relation:the higher bottom echo is, the smaller grain size is.Moreover, products with flaw and even with higher sound speed, higher bottom echo and higher coercivity, will have fewer using time,because the flaw places produce stress concentration which is illustrated in product 6. It is needed showing that the flawed products still have bigger strength because there are no flaws in samples.

The Table 1 shows that the consequences of conducting ultrasonic defection, sound speed measurement, bottom echo measurement and coercivity measurement of products can evaluate the quality of products. The contrast in quality among non-destructive consequences and dissection testing and using situation indicates that this method of non-destructive testing can pledge the quality of products.

number	defecting results	soundspeed CL (m/s)	bottom echo highnees (gain dB)	coercivity Hc (KA/m)	grain degree (mm)	strength (MPa)	using consequence
1	flawful	6684.0	43	8.6	1.6- 2.0	1021	using once was crushed
2	flawless	6679.7	42	8.46	1.6- 2.6	1019	using once was crushed
3	laminated flaw	6762.5	41	15.02	1.0- 2.0	2065	using once was crushed
4	flawless	6731.9	34	14.49	0.8- 1.0	2130	using 3040th no problem
5	flawless	6758.3	33	13.49	0.8- 1.0	2097	using 1859th no problem
6	flawful	6775.1	34	15.89	0.8- 1.0	2053.3	using thrice was crushed
Table 1:

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