Cast Iron Metallographic Specimen Preparation Guide

Material Overview

Cast iron is an iron-based alloy that is initially in a cast state and cannot withstand industrial deformation processing at any solid temperature. This definition can better define "steel" and "cast iron".

Industrial cast iron is a complex iron-based alloy based on Fe-C-Si, with w (C) in the range of 2% to 4.0%, in addition to elements such as manganese, phosphorus, and sulfur. In order to improve and strengthen certain properties of cast iron, elements such as copper, nickel, molybdenum, chromium, and vanadium are often added to form alloy cast iron. The metallographic structure of cast iron is composed of graphite and matrix.

The common structures of cast iron are pearlite, pearlite-ferrite and ferrite, as well as cementite and phosphorus eutectic. By using different heat treatments or castings after adding alloying elements, structures such as bainite, martensite, and ledeburite can be obtained. It can be seen that under different conditions, the matrix structure of cast iron can vary in a wide range.

It can be seen that under different conditions, the matrix structure of cast iron can vary in a wide range.

There are many ways to classify cast iron. According to the existence state of carbon in cast iron and the form of graphite, it can be divided into five categories: white cast iron, gray cast iron, ductile iron,compacted graphite iron and malleable cast iron.。

Preparation Guide

1、Cutting

Alloy white cast iron is very hard (HV6 0 0). Regardless of its hardness, diamond cutting wheels are not suitable for cutting white cast iron. It is recommended to cut by cubic boron nitride grinding wheels. For cutting cast iron with graphite, it is recommended to choose aluminum oxide cutting discs according to the hardness of the cast iron to be cut. For large-scale cutting, automatic cutting is more efficient than manual cutting.

Alumina resin cutting disc SmartCut-10 manual and automatic cutting machine

2、Mounting

Hot inlay is recommended for cast iron inlay. For soft and medium-hard cast iron, UNIWARM universal hot inlay resin is recommended; for harder cast iron, EDGWARM edge-preserving hot inlay resin is more suitable.

Edge-protection Hot Mounting Resin Penetration-20 Auto Dual-station Hot Mounting Machine

3、Grinding and polishing

Graphite is soft and difficult to maintain its actual shape and size. Ferrite and/or austenite cast iron matrix is more prone to deformation and scratches.

GD-Dia Diamond Magnetic Grinding Disc Metallographic Sandpaper

Harder white cast iron and ADI cast iron can be coarse ground with diamond grinding discs (GD-Dia 220#) and fine ground with diamond grinding discs GD-Sof. Soft and medium-hard cast iron (with ferrite, austenite or pearlite matrix) can be coarse ground with silicon carbide sandpaper and fine ground with diamond grinding discs GD-Sof. Diamond grinding discs can keep the sample flatter than silicon carbide sandpaper without protruding graphite.

Alcohol-based diamond suspension Polishing lubricant coolant

It is recommended to use anhydrous alcohol-based diamond suspension and alcohol-based polishing lubricant coolant because cast iron is easily oxidized during polishing,

1、White cast iron

The step of grinding & polishing		Coarse Grinding	Fine Grinding	Diamond Coarse Polishing	Diamond Fine Polishing	Diamond Final Polishing
1	Surface	GD-Dia	GD-Sof	PCM canvas	PCY Wool fabric	PCV Red Short velvet
2	Abrasive Granularity	220#		9μm	3μm	1μm
3	Polishing/ Lubricant	Alcohol, polishing lubricantcoolant	Alcohol, polishing lubricantcoolant	DS001 High performance polycrystalline diamond suspension	DS001 High performance polycrystalline diamond suspension	DS001 High performance polycrystalline diamond suspension
4	Pressure	30N	25N	25N	20N	15N
5	Rotation Speed	300	300	150	150	150
6	Time/'min.	3	4	3	2~5	2~3

2、Cast iron with graphite

The step of grinding & polishing		Coarse Grinding	Fine Grinding	Diamond Coarse Polishing	Diamond Fine Polishing	Diamond Final Polishing
1	Surface	SiC sandpaper	GD-Sof	PCY Wool fabric	PCV Red Short velvet	PCX Rubber cloth
2	Abrasive Granularity	400#		3μm	1μm	0.05μm
3	Polishing/ Lubricant	water	water	DS001 High performance polycrystalline diamond suspension	DS001 High performance polycrystalline diamond suspension	OS001-JP aluminum oxide polishing liquid
4	Pressure	30N	25N	25N	20N	15N
5	Rotation Speed	300	300	150	150	150
6	Time/'min.	2	4	3	2~5	2~3

The above recommended parameters are taken from: Metallographic preparation method of Ø30 mm cast iron with Ø250 mm working plate on automatic grinding and polishing machine Smoothneer-6.

Etching

Graphite-containing cast iron is not suitable for electrolytic polishing because the graphite will be etched away by the electrolyte. However, if the microstructure of the substrate needs to be quickly identified, electrolytic polishing and etching can be used

Common cast iron etching agents are as follows:

Composition, usage and instructions of commonly used etchants for cast iron

No.	Composition	Usage and Instructions
1	Nitric acid 0.5~6.0ml Ethanol 96~99.5mLL	The cast iron matrix structure is displayed, and the etching time is from afew seconds to 1 minute. For high-dispersion structures, low-concentration solutions can be used for etching to slow down thecorrosion rate and thus improve the clarity of the structure.
2	Picric acid 3~5g Absolute ethanol 100mL	Shows the cast iron matrix structure. The corrosion rate is relatively slow,. and the erosion time is from a few seconds to a few minutes.
3	Copper chloride 1g Magnesium chloride 4g Hydrochloric acid 2mL Anhydrous ethanol 100mL	Display the interface of cast iron eutectic group. Use absorbent cotton todip the solution and evenly apply it on the polished surface of thesample. The etching speed is slow and the effect is good.
4	Copper chloride 3g Ferrous chloride 1.5g Nitric acid 2mL Anhydrous ethanol 100mL	Shows the interface of cast iron eutectic, and the etching speed is faster

Organization Appreciation

Ductile Iron 200X

Ductile Iron 50X

Gray cast iron after corrosion 100X

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