Metallographic Preparation of Welded Parts Guide

Material Overview

Because welding is simple, economical, safe and can simplify the manufacturing process of parts with complex shapes, welding technology has been widely used in the machinery manufacturing industry. Many riveted structures in the past have also been replaced by welded parts. Therefore, the application of welding technology will become more and more extensive, and the metallographic inspection of welded parts will also increase. Many types of manufacturing industries widely use various welding processes. These manufacturing industries include:

Aerospace industry, such as the manufacture of wings and fuselages

Shipbuilding and navigation industry, such as the manufacture of decks and deck superstructure plates

Land transportation and automobile industry

Welding metallography mainly inspects the structure of welded joints. Welded joints are composed of three parts: weld metal (referred to as weld), heat-affected zone of parent material and non-heat-affected zone of parent material. Since the structures of the three zones on the welded joint are different, metallographic inspections should be carried out separately.

Preparation Guide

Key points in sample preparation:

1、Cutting

In order to avoid changes in the micro-structure and properties of the welded parts caused by thermal damage, any type of thermal damage must be avoided during the cutting process. In order to minimize the deformation caused by cutting and avoid thermal damage to the cut surface, it is very important to take the following measures

Suitable cutting disc

Suitable feed speed

Cutting cooling in place

Silicon carbide resin cutting disc Cutting cooling lubricant

2、Mounting

Depending on the welding materials and flux, cold mounting is used for materials that are sensitive to pressure and temperature. Otherwise, hot mounting can be used to ensure better mounting quality. For mounting of larger welded parts, the silicone cold mounting mold of Truer can be used, which has an inner diameter capacity of L*W*H: 100x50x22mm.

MC003 Vacuum-free Clear Cold Mounting Kit

Cold Mounting Mold

3、Grinding and polishing

Due to phase changes during welding or due to the presence of different metals in the welding parts, welding parts specimens may have wide differences in material hardness. The weld metal may contain harder precipitates or some inherent welding defects. Therefore, it is very important to use appropriate preparation methods to ensure that the micro-structure has minimal protrusions and maintains all the basic elements of the micro-structure. In this case, it is best to use automatic or semi-automatic preparation equipment because it can ensure polishing consistency and repeatability, thereby ensuring the accuracy of micro-structural analysis.

The following are several common metallographic preparation methods for welding materials:

1、Low-carbon structural welding parts

The step of grinding & polishing		Coarse Grinding	Fine Grinding	Diamond Coarse Polishing	Diamond Coarse Polishing	Diamond Final Polishing
1	Surface	GD-Dia Diamond MagneticGrinding Disc	GD-Sof Composite MagneticGrinding Disc	PCM canvas	PCY Wool fabric	PCV Red Short velvet
2	Abrasive Granularity	220#		9μm	3μm	1μm
3	Polishing/ Lubricant	water	water	DS001 High performance polycrystalline diamondsus pension	DS001 High performance polycrystalline diamondsus pension	DS001 High performance polycrystalline diamondsus pension
4	Pressure	35N	30N	25N	25N	20N
5	Rotation Speed	300	300	150	150	150
6	Time/min.	2	4	2	3~5	2~3

2、Stainless steel welded parts

The step of grinding & polishing		Coarse Grinding	Fine Grinding	Diamond Coarse Polishing	Diamond Coarse Polishing	Diamond Final Polishing
1	Surface	SiC sandpaper	GD-Sof Fine Grinding Disc	PCM canvas	PCC acetic acid	PCE Long velvet
2	Abrasive Granularity	320#		9μm	3μm	1μm
3	Polishing/ Lubricant	water	water	DS001 High performance polycrystalline diamondsus pension	DS001 High performance polycrystalline diamondsus pension	DS001 High performance polycrystalline diamondsus pension
4	Pressure	30N	30N	25N	25N	20N
5	Rotation Speed	300	300	150	150	150
6	Time/min.	2	4	2	3~5	2~3

3、Titanium and titanium alloy welded parts

The step of grinding & polishing		Coarse Grinding	Fine Grinding	Diamond Coarse Polishing	Diamond Coarse Polishing	Diamond Final Polishing
1	Surface	SiC sandpaper	SiC sandpaper	PCY Wool fabric	PCE Long velvet	PCX Neoprene
2	Abrasive Granularity	320#	800#、1200#、4000#	3μm	1μm	0.04μm
3	Polishing/ Lubricant	水	水	DS001 High performance polycrystalline diamondsus pension	DS001 High performance polycrystalline diamondsus pension	SiS001-JP Silicon dioxide polishing liquid
4	Pressure	30N	25N	25N	20N	15N
5	Rotation Speed	300	300	150	150	150
6	Time/min.	1	2	3~5	2~3	2~5

The above recommended parameters are taken from: Preparation method of 30 mm Ø welding sample with 250 mm Ø working plate on automatic grinding and polishing machine Smoothneer-6.

Etching

The macroscopic cross section after etching can identify the metal boundary of the weld, the heat-affected zone, the fusion boundary, the grain growth, each pass of the multi-pass weld, and welding defects such as cracks, pores, holes, incomplete penetration and lack of penetration.

Appendix: Names, compositions and uses of commonly used etchants for welds

No	Name	composition	Usage Range	Remark
1	4% nitric acid aqueous solution	Nitric acid: water (volume ratio 4:100)	Carbon steel, low alloy steelcarburized parts, welded parts and cladding layers	When preparing the etchant, it isnecessary to add the components strictly according to the order of theomponents.In hot summer, it is besto prepare it in a cold water bath toavoid excessive reaction when addingthe chemical test cobalt-based material cladding agent
2	Ferric chloride hydrochloric acid aqueoussolution	Ferric chloride: hydrochloric acid: water （0g:100mL:50mL）	Welding of stainless steel, R45HK40 and other heat-resistantsteel, copper and copper alloys
3	Oxalic acid aqueous solution	Oxalic acid: water (10g:100mL) 3 1.5~6V,30~60s	Stainless steel, crMnN, Ni7N.HK40, R45,C25Ni20 and otheheat-resistant steel welding parts
4	Picric acid hydrochloric acid alcohol solution	Picric acid: hydrochloric acid:alcohol	High carbon and highchromium wear-resistantmaterial surfacing layer

Organization Appreciation

Brass + Copper and Silver Welding Rod 50X Before Corrosion

Chip solder joint length measurement 200X

316L stainless steel + nickel-based alloy + silver welding rod 200X

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