Valve Metallographic Analysis

Valve Metallographic Analysis

Metallographic analysis is one of the important means of experimental research on metal materials. Using the principle of quantitative metallography, the three-dimensional space morphology of the alloy structure is determined by the measurement and calculation of the metallographic microstructure of the two-dimensional metallographic sample’s polished surface or thin film. Quantitative relationship between alloy composition, structure and properties.

(1) Selection of metallographic samples

Longitudinal specimens: Take samples along the forging direction of the steel. Main inspection: deformation degree of non-metallic inclusions, grain distortion degree, plastic deformation degree, various microstructures after deformation, overall situation of heat treatment, etc.

Transverse sample: Take the sample perpendicular to the forging and rolling direction of the steel. Main inspection: the structure, grain size, carbide network, surface defect depth, oxide layer depth, surface chemical heat treatment and coating thickness of metal materials from the surface layer to the center.

Defect or Failure Analysis Sampling: Sampling shall include the defective portion of the part. For example, it includes the fracture at the time of fracture, or takes the cross section of the crack to observe the depth of the crack and the changes in the surrounding structure. When sampling, care should be taken not to damage or disappear the defects during grinding.

The sample can be hand saw, grinder, microtome, EDM, etc., no matter what method is used, it cannot cause structural changes due to deformation or overheating.

(2) Mounting of metallographic samples

Mechanical inlay method: The sample is placed in a steel ring or a small steel clamp and fixed with bolts and spacers.

Resin mounting method: use resin to mount small metallographic samples. It can be divided into two categories: hot pressing and casting inlay.

(3) Grinding of metallographic samples

A. Smoothing is rough grinding: use a grinder or sandpaper to smooth the sample, and pay attention to cooling.

B. Polishing is fine grinding: by grinding from coarse to fine sandpaper, the deep grinding marks on the surface and the surface processing deformation layer are removed. Every time you change the sandpaper, turn it 90 degrees to remove the scratches on the road. Metallographic sandpaper abrasives are generally silicon carbide and aluminum oxide. Watch out for cooling.

C. Polishing: remove the fine wear marks and surface deformation layer, so that the grinding surface becomes a smooth mirror surface without scratches. There are mechanical polishing and electrolytic polishing.

Mechanical polishing: commonly used diamond abrasive paste.

Electrolytic polishing: the use of electrochemical dissolution to achieve the purpose of polishing the surface.

(4) Erosion of metallographic samples

A. Chemical erosion: The dissolution of chemical reagents is used to reveal the organization of metals by means of chemical or electrochemical action.

Immersion and wipe methods.

When the bright surface gradually loses its luster and finally becomes gray-black, it is generally advisable to corrode shallowly.

B. Electrolytic erosion: the potential difference between the phases. It is suitable for alloys with extremely high chemical stability that are not easily corroded by chemical attack methods, such as heat-resistant steel, thermocouple materials, etc.

Relevant testing standards

Microstructure test method of GB/T13298-91 steel

Microstructure evaluation method of GB/T13299-91 steel

GB/T10561-2005 Determination of the content of non-metallic inclusions in steels

GB/T11354-2005 Determination of nitriding layer depth and metallographic structure inspection of steel parts

JB/T7710-2007 Microstructure inspection of thin-layer carbonitriding or thin-layer carburizing steel parts

JB/T7713-2007 Microstructure inspection of high carbon and high alloy steel cold working die

JB/T9211-2008 Martensitic grades of medium carbon steel and medium carbon alloy structural steel

JB/T8420-2008 hot work die steel microstructure rating

GB/T7216-2009 Metallographic inspection of gray cast iron

GB/T9441-2009 Metallographic inspection of ductile iron

JB/T9204-2008 Induction hardening metallographic inspection of steel parts

GB/T14979-1994 Evaluation method of eutectic carbide inhomogeneity of steel

GB/T224-2008 Decarburization layer depth determination method of steel

GB/T6394-2002 Determination method of average grain size of metals

GB/T6462-2005 Microscopic method for thickness measurement of metal and oxide coatings