High Speed Steel’s Quality Inspection and Defected Analysis

Recently , there have several client consult with the material ‘s quality inspection issue .

Specially for M2, M42 , M50 ... Round bar .

The quality inspection of High Speed Steel including two parts with raw material  and heat treatment.

The inspection of raw material including chemical component , size & shape , macro-structure & fracture ，eutectic carbide heterogeneity , decarburization , surface quality and large pieces carbide ,etc .

The inspection of heat treatment including hardness （Hardness after annealing and quenching and tempering），distortion ,micro-structure (after the tempering , annealing , quenching )..

The inspection of raw material：

1）Chemical Component according with the rules of GB/T9943

According with the steel grade’s standard to take the sample for chemical testing .

All the chemical component should under the range of value, on the contrary , it should return the item back to the supplier .

2）Surface Quality according with the rules of GB/T9943

After the pressure under the process of hot rolled or forged in the production , the item’s surface maybe will have some crack part , folding , scar , cointaminating particle ....it should be clear it out .The clearance ‘s depth will calculate from the actual steel ‘s specification, generally the cross-selection ＜80mm , the partly defected depth should be allow to the half of tolerance ,if the steel size between 80-120mm  ,the partly defected depth should be allow to the tolerance,the clearance ‘s part not less the 5 times of the depth .

The others sightly defected part where it is under the tolerance ‘s range it can allow not to clear it out .

The steel with hot rolled / forged applying to the cutting process ,the steel ‘s section dimension ＜80mm, the surface allow to have the partly defected but the depth can not more than the half of tolerance from the Nominal dimensions of steel .

The steel’s sectional dimension with 80-120mm, the surface allow to have the defected part which depth not more than to the tolerance of 80-120mm .

The cold drawn steel ‘s surface should be clean , smooth , not any cracks, wrinkles, scarring, folds, inclusions and oxide sheets .

After the annealed process , the cold drawn steel allow to have the color of Oxidation  or light oxide layer.

The steel surface allow to have the defection such as pitting, individual scratches, concave surfaces, chalkboard and lubricant marks which depth not more than its specification’s tolerance .

3)Low - fold structure and fracture

3）The steel ‘s Low - fold structure can not have the contraction cavity ,bubble ,  skull patch and slag under to check by the human eyes according with the rules of GB/T9943. The size of steel ≤120mm , its Central porosity, general porosity and segregation with pass class not more than class I .

The steel can not allow to have the Naphthalene fracture .

If can not to check the Naphthalene fracture if the supplier can guarantee the steel without the Naphthalene fracture.

 4)High Speed Steel’s Specification and The Pass Class of Carbide Heterogeneity as the below sheet .

Tungsten series high speed tool steel shall be rated according to the first rating column (divided into two groups) of GB/T14979, tungsten and molybdenum series high speed tool steel shall be rated according to the second rating column  (divided into two groups), larger than 120mm shall be rated according to the third rating column .

   High Speed Steel’s Specification and The Pass Class of Carbide Heterogeneity           

P.S.The assessment of Glass of carbide heterogeneity of steel ,under the rules of heat treatment , the quenching process should under the temperature of 680-720℃to do the tempering process for 1-2 hours and then taking the sample with thickness 10-12mm from the cross direction , if the steel with bar , it should take the sample from the diameter or one quarter of the way across to test the portrait part’s heterogeneity .

5) The large block of carbide

Large block carbides of high speed tool steel are graded according to GB/T4462 standard. Tungsten high speed steel  shall be evaluated according to the first grade column , and molybdenum high speed steel  shall be evaluated according to the second grade column. Qualified grades of tungsten high speed steel  are shown below. For molybdenum high-speed steel wire, the qualified grade shall not be greater than grade 4. Other steel shall be specified in the corresponding technical conditions.

                 The Pass Classification of Carbide Heterogeneity

P.S.

 1)The inspection of the large block of carbide of Tungsten series high speed tool steel, it should take the sample with thickness 10-12mm from the portrait selection of quarter of catercorner of diameter . And the sample should do the quenching and tempering under the temperature under 680-700℃for 1-2 hours , magnifying with 500 times for checking it .

2) To check the large particles of Tungsten series high speed tool steel,it should take the sample from the steel with annealed condition from the crosswise .

6)Decarburizatioin

Decarburization layer includes total decarburization and partial decarburization (ferrite + transition layer) depth sum。

According to the rules of GB/T 9943 ,hot rolled /forged steel , Tungsten series high speed tool steel’s Unilateral decarburization layer is less than or equal to 0.30+1.0%D, TUNGSTEN and molybdenum series steel is less than or equal to 0.40+1.30%D, W9Mo3Cr4V steel is less than or equal to 0.35+1.1%D; The cold draw steel ,Tungsten series steel ≤1.0%D,TUNGSTEN and molybdenum series steel is less than or equal to 1.3%D. D means the steel’s thickness or diameter .

3. The inspection of quality of heat treatment

(1) Visual Inspection

With the naked eye or magnifying glass to observe the surface of the working piece cracks, pitting, collision and other macro defects.

(2) Hardness Inspection

①Hardness of returned goods shall be checked by Brinell hardness tester, hardness of quench shall be checked by Rockwell hardness tester.

The hard part of the knife edge with standard file inspection, the file slip as qualified.

The hardness of the tool after quenching should be greater than HRC60, otherwise, it needs to be redone and re-hardened after annealing. The hardness after tempering should meet the requirements of the drawing. The hardness after annealing shall meet the hardness requirements of raw materials supplied to customers, and the thermal hardness shall be checked on the high temperature hardness tester.

② It should test for 3 times in the same part to check its hardness .No allow to have the soft point for the important and micro tools. Tools with cross section sizes greater than 80MM can allow to have a small number of soft points (1%-2%), but the hardness value of the soft point shall not be below hrc3-5, the lower limit of the technical requirements.

③Test the hardness of push, broach front guide and middle teeth. There are two soft points that need to be redone or scrapped by the supplier. The hardness of a tool with a prefab tail should be tested too .

④Local heating quenching tool, the section size is less than 50mm, quenching hardness area size tolerance of +/-10mm;

If the section size is greater than 50mm, the allowable deviation is +/-20mm.

⑤Mass production for tooling steel to test its hardness should be check with 3%-5% of totally quantity ; Important tools with complex shapes such as broaches, hobs, etc. should be 100% tested.

（3）Distortion detection

①Long rod tools should detect its bending, it should do be straightened if the bending ‘s value than the allowable value from the drawing .

②Flat cutter (saw blade milling cutter, circular saw blade) flatness check, flatness out of tolerance, leveling; Roundness inspection is also needed. If roundness exceeds the grinding allowance, it will be scrapped.

③Gear cutting tool should check the size between teeth, check if the teeth have  distortion or not .

(4) Micro-instruction Inspection

①Micro-instruction refers to quenched grain size, degree of tempering, degree of overheating.

②According to ZBJ 36003-1987 "tool steel metallo-graphic examination for heat treatment", the standard picture of quenching grain size grade of W18Cr4V steel at 450 times of magnification has eight grades: 11.5, 11,10.5, 10,9.5, 9,8.5, 8.

W6Mo5Cr4V2 steel has 7 grain sizes, 12,11,10.5, 10,9.5, 9,8.5,

③According to the provisions of ZBJ 36003-1987, the quenching degree of W18Cr4V steel is 5 levels, the overheating degree of W6Mo5Cr4V2 steel is 4 levels, and the tempering degree is 3 levels, that is, 1 to 3 levels (amplification of W18Cr4V 500 times, amplification of W6Mo5Cr4V2 450 times). When examining the degree of overheating, at least three visual fields should be observed, and the grade should be judged on the basis of the worst visual field.

④The technical requirements for micro-structure levels of various high speed tool steels are shown in the following table.

The technical requirements for micro-structure levels of various high speed tool steels

P.S.

①The degree of superheat is less than or equal to 3 in cobalt carbide accumulation.

②Gear shaver allowed overheating level ≤1.

⑤When checking quenching grain size, alcohol solution with volume fraction of nitric acid of 4% or aqueous solution with mass fraction of ferric chloride of 5%-10% is usually used as corrosive agent. When checking the degree of overheating, alcohol solution with volume fraction of nitric acid of 4% was used as the corrosive agent. To check the degree of tempering, use 4% nitric acid alcohol solution + 2% picric acid alcohol solution + 5% hydrochloric acid alcohol solution as the etch agent. When the temperature of etchant is 10-18℃, the corrosion lasts 2.5-3 minutes. When the temperature of etchant is 26-30℃, the corrosion lasts 1.5-2 minutes. When the temperature is higher than 30℃, the corrosion time is less than 1 minute.

⑥Inspection of the uneven degree of steel carbide:

Take W18Cr4V as an example, heat to 1270-1280℃, hold for 1.5-2 minutes, oil quenching; Reheat to 680-700℃ and temper for 1 hour. The purpose of quenching is to dissolve most of the secondary carbides so as not to confuse them with eutectic carbides when observed. Tempering makes quenched martensite which is difficult to corrode into tempered martensite which is easy to corrode into black, so that the carbide is clearly visible in metallo-graphic field.

⑦Table of influence of different test positions on carbide rating

 P.S. The sample is fully ground, eliminate all decarbonization layer, corrosion is not excessive, in order to display clearly is appropriate.

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