Determination of Free Cementite, Pearlite and Widmanstaten Structure in Steel
1 Scope
This document specifies the cutting and preparation of test piece and evaluation methods of free cementite, pearlite and Widmanstaten structure in steel.
This document is applicable to the evaluation of free cementite, pearlite and Widmanstaten structure in low carbon and medium carbon steel plates, strips and rods (billets). Other steel types may refer to this document.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 13298 Inspection methods of microstructure for metals
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
ferrite
α-Fe containing alloying elements in solid solution
[Source: GB/T 30067-2013, 2.2.247]
3.2
pearlite
a metastable microstructure formed, when local austenite areas attain the eutectoid composition, in alloys of iron and carbon containing greater than 0.025% but less than 6.67% carbon. The structure is an aggregate consisting of alternate lamellae of ferrite and cementite formed on slow cooling during the eutectoid reaction
[Source: GB/T 30067-2013, 2.2.348]
3.3
free cementite
a granular cementite that exists independently on a ferrite matrix or grain boundary
Note: The free cementite in low carbon annealed steel with carbon content less than 0.15% is mainly the product of pearlite transition, in which there may be a small amount of tertiary cementite; for very low carbon (carbon content less than 0.02%) steel, free cementite is tertiary cementite.
3.4
pearlite colony
a circumscribed aggregate within which lamellae of corresponding phases have the same orientation
[Source: GB/T 30067-2013, 2.2.349]
3.5
Widmanstaten structure
in hypoeutectoid steel, the preeutectoid ferrite grows from the austenite grain boundary or within the grain along the austenite plane into the grain, precipitates out in acicular shape, and there is pearlite structure in it
[Source: 2.2.268, GB/T 30067-2013, modified]
4 Cutting and preparation of test piece
4.1 Selection of test piece
4.1.1 Cutting position
In general conditions, test pieces shall be cut on the steel (or intermediate billet) in delivery state, and the sampling method and quantity shall be in accordance with the product standard or technical conditions. If there are no special requirements, the inspection surface of free cementite and pearlite test pieces should be parallel to the longitudinal axis of steel strip, steel plate and steel rod (billet), and the inspection surface of Widmanstaten structure test pieces should be perpendicular to the longitudinal axis of steel strip, steel plate and steel rod (billet). The specific location of sampling method as suggested in Figures 1 to 6 (Position A is suitable for the sampling of Widmanstaten structure test pieces; Position B is suitable for sampling of free cementite and pearlite test pieces).
4.1.2 Sampling methods of steel plates or strips
Sampling methods of steel plates or strips are as follows:
a) Steel plates or strips with thickness less than or equal to 25 mm: test pieces should be cut at the middle part from the center of steel plates (strips) to the outer surface. The inspection surface should be the full thickness section (see Figure 1), and the length of the sample is recommended to be no less than 20 mm.
b) Steel plates or strips with thickness greater than 25 mm and less than or equal to 50 mm: test pieces should be cut at the middle part from the center of the steel plates (strips) to the outer surface. The inspection surface is 1/2 section of the thickness of the steel plate (see Figure 2), and the inspection area should not less than 200 mm2;
c) Steel plates or strips with thickness greater than 50 mm: test pieces should be cut from the middle part between the center of steel plates (strips) and the outer surface (see Figure 3), and the inspection area should not less than 200mm2.
Figure 1 Sampling of steel plates with thickness less than or equal to 25 mm
Figure 2 Sampling of steel plates with thickness greater than 25 mm and less than or equal to 50 mm
Figure 3 Sampling of steel plates with thickness greater than 50 mm
4.1.3 Sampling methods of steel bars or billets
Sampling methods of steel bars or billets are as follows:
a) Steel bars or billets with diameter or side length less than or equal to 25 mm: test pieces should be cut through the diameter of the entire section (see Figure 4), and the length of test pieces is recommended to be no less than 20 mm;
b) Steel bars or billets with diameter or side length greater than 25 mm and less than or equal to 40 mm: test pieces should be cut at the middle part from the center of steel bars (billets) to the outer surface. The inspection surface should be half of the section through the diameter (from the center of test piece to the edge, as shown in Figure 5), and the inspection area should not be less than 200 mm2;
c) Steel bars or billets with diameter or side length greater than 40 mm: test pieces should be cut at the middle part from the center of the steel bars (billets) to the outer surface. The inspection surface should be a partial section from the outer surface of the steel to the central position (see Figure 6), and the inspection area should be not less than 200 mm2.
Figure 4 Sampling of steel bars or billets with diameter or side length less than or equal to 25 mm
Figure 5 Sampling of steel bars or billets with diameter or side length greater than 25 mm and less than or equal to 50 mm
Foreword II
1 Scope
2 Normative references
3 Terms and definitions
4 Cutting and preparation of test piece
5 Evaluation method of free cementite
6 Evaluation method of pearlite
7 Evaluation method of Widmanstaten structure
8 Test report
Annex A (Nominative) Standard rating chart of free cementite
Annex B (Informative) Example of quantitative analysis of cementite in SPCC steel
Annex C (Nominative) Standard rating chart of pearlite
Annex D (Nominative) Standard rating chart of Widmanstaten structure
Bibliography
