GB/T 224-2019Determination of the depth of decarburization of steels (English PDF)

3.1 decarburization

Loss of carbon on steel surface. Such loss of carbon includes:

a) partial decarburization: carbon content on surface of steel sample is

reduced below carbon content of substrate, and is greater than solid

solution limit of carbon in ferrite at room temperature;

b) complete decarburization: is also called as ferrite decarburization; surface

carbon content of sample is lower than maximum solid solubility of carbon

in ferrite; only ferrite exists.

NOTE: Complete decarburization described in b) has only ferrite structure. It shall use

metallographic method to determine.

3.2 depth of functional decarburization

Distance from sample surface to point of specified carbon content or point of

hardness level. Specified carbon content or hardness level shall be based on

that use performance is not affected by decarburization (for example, minimum

carbon content specified in product standard).

3.3 depth of total decarburization

Distance from sample surface to the point of which carbon content is equal to

substrate carbon content. It is equal to the sum of partial decarburization and

complete decarburization.

See Figure 1 for different decarburization zones.

W - width;

A - area of inspection surface of steel plate or rectangular sample.

Figure 4 -- Typical sampling method for steel plate and rectangular steel

4.4 Sample for chemical analysis shall have a sufficient length, so as to ensure

quantity of turnings that are continuously increased by chemical analysis. Or it

shall have a large enough area to meet spectral analysis area.

5.1 General

5.1.1 Selection of determination method and its precision depend on product's

degree of decarburization, microstructure, carbon content as well as part's

shape. Final product usually uses following methods to measure depth of

decarburization:

a) metallographic method (see 5.2);

b) microhardness method (see 5.3); it can use Vickers hardness or Knoop

hardness;

c) determination method for carbon content can be chemical analysis method

(see 5.4.2), direct spectroscopy reading method (see 5.4.3), electron

probe method (see 5.4.4) or glow spectroscopy method (see 5.4.5).

5.1.2 Determination methods in 5.1.1 all have their own application scopes. The

method for determination shall be determined by product standard or agreed

by both parties. Where it is not specified, it shall use metallographic method.

5.2 Metallographic method

5.2.1 General

This method is, under optical microscope, to observe structure change of

sample from surface to substrate as carbon content varies. It is applicable to

type of steel that has annealing or normalizing (ferrite-pearlite) structure. It can

be conditionally used for those hardened, tempered, rolled or forged products.

5.2.2 Sample preparation

Sample is ground and polished according to GB/T 13298. However, rounding

and curling are not allowed on sample edge. For this purpose, sample can be

mounted or fixed in holder. If necessary, surface of testing sample can be plated

with a layer of metal to protect it. Automatic or semi-automatic sample

measurements at random (at least five). Take average value of these

measurement values as depth of total decarburization. For bearing steel, tool

steel, spring steel, measure depth of total decarburization at the deepest.

Averaging method measures the first point at the deepest uniform

decarburization zone. Then, starting from this point, surface is equally divided

into several parts. If there are no special regulations, at least it shall be divided

into four equal parts. At the end position of each part, measure depth of

decarburization at the deepest. Take average value of these measurements (at

least four) as depth of total decarburization and report it. Averaging method is

applicable to steels such as bearing steel, tool steel, spring steel that have high

requirements for decarburization.

If there are no special provisions in product standard or technical agreement,

when measuring, those deep decarburization points (such as cracks or folds,

workpiece corners, etc.) in sample caused by steel defects shall be excluded

(but defects shall be noted in test record).

5.2.3.2 Determination of complete decarburization

Determination method of complete decarburization is same with 5.2.3.1.

5.2.3.3 Determination of functional decarburization

Determination method of functional decarburization is same with 5.2.3.1.

Judgement of functional decarburization is determined in product standard or

agreed by parties concerned.

5.2.3.4 Typical structural photo of decarburization depth determined by

metallographic method

Refer to Annex A for typical structural photo example of decarburization depth

determined by metallographic method.

5.3 Microhardness method

5.3.1 General

This method is to measure distribution gradient of microhardness value in cross

section that is perpendicular to surface of sample.

This method is only applicable to hardened, tempered or heat treated eutectic

steel of which decarburization is rather deep. Meanwhile, it is also used for

situation where decarburization is completely in hardened zone to avoid

hardness fluctuation caused by incomplete quenching. This method is not

applicable to low-carbon steel.

Depth of decarburization is specified as distance from surface to the point that

has reached required hardness value (dispersion of measurement needs to be

considered into estimation).

In principle, it shall perform four groups of measurements at a position where

they are as far as possible from each other. Use average value of their

measurements as depth of decarburization.

Measurements limit of depth of decarburization can be:

a) from sample surface to position where hardness value is specified in

product standard or technical agreement;

b) from sample surface to position where hardness value is stable;

c) from sample surface to a certain percentage of position where hardness

value is stable.

Which kind of measurement limit used is specified in product standard or

agreement of both parties.

5.4 Determination method of carbon content

5.4.1 General

This method is to determine distribution gradient of carbon content in a direction

perpendicular to surface of sample.

5.4.2 Chemical analysis method

5.4.2.1 General

This method is applicable to sample that has proper geometry, for example,

cylinder or polyhedron with planar body.

NOTE 1: Sample is usually not heat-treated. If it is really necessary, after proper

consultation, appropriate heat treatment can be carried out, but it is guaranteed not to affect

depth of decarburization.

NOTE 2: This method is not applicable to partial decarburization.

5.4.2.2 Sample preparation

Use machining method to, parallel to surface of sample, strip sample chips of

which each layer is 0.1mm thick. Take care to prevent any pollution. Remove

oxide film in advance. Collect metal sample chips stripped from each layer.

Determine carbon content in accordance with GB/T 20126 or ISO 9556.

Determination of depth of total decarburization is from surface to position where

carbon content is stable. Depth of complete decarburization is according to

agreement between supplier and purchaser. Conduct four groups of

determinations at a position where distance between each other is as far as

possible. Take average value of determination values as depth of

decarburization.

5.4.5 Glow spectroscopy method

5.4.5.1 General

This method is in accordance with GB/T 19502 and GB/T 22368. This method

is applicable to round sample of which depth of decarburization does not

exceed 100µm, diameter is between 20mm ~ 100mm or square sample of

which side length is between 20mm ~ 100mm. Plane size of sample shall meet

requirements for glow spectrometer.

5.4.5.2 Sample preparation

Use suitable solvent (analytically-pure acetone or ethanol) to rinse sample

surface to remove oil or other residues. During rinsing, it can use wet soft non-

woven fabric to gently wipe sample surface. After rinsing, it can use inert gas

(argon or nitrogen) or clean oil-free compressed air to blow sample dry. Gas

piping must not be in contact with surface of sample.

5.4.5.3 Measurement

Depth curve of carbon content goes from sample surface to position where

substrate carbon content is stable. Depth of total decarburization is determined

by depth curve of carbon content.

Determination of depth of total decarburization is from surface to position where

substrate carbon content is stable, for example, to position where substrate

carbon content is. Depth of total decarburization can be according to agreement

between supplier and purchaser. Conduct four groups of determinations at a

position where distance between each other is as far as possible. Take average

value of determination values as depth of decarburization.