How to Heat Treat Gray Cast Iron?

1. Annealing

1. Stress relief annealing

In order to eliminate the residual stress of the casting, stabilize the geometric size, reduce or eliminate the distortion after cutting, it is necessary to conduct stress relief annealing on the casting.

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How to Heat Treat Gray Cast Iron? 1

The chemical composition of cast iron must be considered in determining stress relief annealing.

When the temperature of common gray cast iron exceeds 550 ℃, graphitization and granulation of some cementite may occur, which reduces the strength and hardness.

When the alloy element is contained, the temperature at which the cementite begins to decompose can be increased to about 650 ℃.

Generally, the stress relief annealing temperature of ordinary gray cast iron is 550 ℃, that of low alloy gray cast iron is 600 ℃, that of high alloy gray cast iron can be increased to 650 ℃, and the heating speed is generally 60 ~ 120 ℃.

The holding time determines the heating temperature, the size and structural complexity of the casting and the requirements for stress relief.

The following figure shows the relationship between holding time and residual stress under different annealing temperatures:

How to Heat Treat Gray Cast Iron? 2

Fig. 2 Relationship between annealing temperature and time and residual internal stress

a) Composition (mass fraction) (%): C3.18,Si2.13,Mn0.70,S0.125,PO.73,Ni1.03,Cr2.33,Mo0.65;

b) Composition (mass fraction) (%): C3.12,Si1.76,Mn0.78,SO.097,P0.075,Ni1.02,Cr0.41,Mo0.58;

c) Composition (mass fraction) (%): C2.78,Sil.77,Mn0.55,SO.135,PO.069,NiO.36,CrO.10,Mo0.33,Cu0.46, V0.04.

The cooling speed of stress relief annealing of castings must be slow to avoid secondary stress.

The cooling speed is generally controlled at 20 ~ 40 ℃ / h, and it can be cooled to below 150 ~ 200 ℃ before being discharged for air cooling.

The stress relief annealing specifications of some gray iron castings are shown in the following table:

Table 3 stress relief annealing specification for gray cast iron castings

Casting type

Casting mass / kg

Casting wall thickness / mm

Charging temperature / ° C

Temperature rise rate / (C / h)

Heating temperature / C

Holding time / h/

Slow cooling speed (C / h)

Discharge temperature / C

Ordinary cast iron

Low alloy cast iron

General casting

<200

≤200

≤100

500~550

550-570

4-6

30

≤200

200-2500

≤200

≤80

500~550

550~570

6-8

30

≤200

>2500

≤200

≤60

500-550

550-570

8

30

≤200

Precision casting

<200

≤200

≤100

500-550

550-570

4-6

20

≤200

200~3500

≤200

≤80

500-550

550-570

6-8

20

≤200

Simple or cylindrical castings, general precision castings

<300

10-40

100-300

100-150

500~600

2-3

40-50

<200

100-1000

15-60

100-200

<75

500

8-10

40

<200

Complex structure and high precision casting

1500

<40

<150

<60

420~450

5~6

30~40

<200

1500

40-70

<200

<70

500-550

9-10

20-30

<200

1500

>70

<200

<75

500-550

1.5

30-40

150

Textile machinery small casting machine tool small casting machine tool large casting

<50

<15

<150

50-70

500-550

3~5

20~301

50-200

<1000

<60

≤200

<100

500-550

3-5

20-30

150-200

>2000

20-80

<150

30-60

500-550

8-10

30-40

150-200

2. Graphitization annealing

Graphitization annealing of gray iron castings is to reduce hardness, improve machinability and improve plasticity and toughness of cast iron.

If there is no eutectic cementite in the casting or its quantity is small, low-temperature graphitization annealing can be carried out;

When the amount of eutectic cementite is large, high temperature graphitization annealing is required.

(1) Low temperature graphitization annealing.

The graphitization and granulation of eutectoid cementite will occur when the cast iron is annealed at low temperature, which will reduce the hardness and increase the plasticity of the cast iron.

The low-temperature graphitization annealing process of gray cast iron is to heat the casting to a temperature slightly lower than the lower limit of AC1, keep it for a period of time to decompose the eutectoid cementite, and then cool it with the furnace.

The process curve is as follows:

How to Heat Treat Gray Cast Iron? 3

Fig. 4 low temperature graphitization annealing process curve of gray cast iron

(2) High temperature graphitization annealing.

The high-temperature graphitization annealing process is to heat the casting to a temperature higher than the upper limit of AC1 to decompose the free cementite in the cast iron into austenite and graphite, and then cool it in different ways according to the required matrix structure after holding for a period of time.

If it is required to obtain a ferrite matrix with high plasticity and toughness, its process specification and cooling method are shown in the following figure:

How to Heat Treat Gray Cast Iron? 4

Fig. 5 high temperature graphitization annealing process of ferrite matrix

If it is required to obtain pearlite matrix structure with high strength and good wear resistance, its process specification and cooling method can be carried out according to figure 6 below:

How to Heat Treat Gray Cast Iron? 5

Fig. 6 high temperature graphitization annealing process of pearlite matrix

2. Normalizing

The purpose of normalizing gray cast iron is to improve the strength, hardness and wear resistance of the casting, or as a preliminary heat treatment for surface quenching, to improve the matrix structure.

The normalizing process specification of gray cast iron is shown in the following figure:

Generally, normalizing is to heat the casting to 30 ~ 50 ℃ of the upper limit of AC1, so that the original structure is transformed into austenite.

After holding for a period of time, the casting is discharged for air cooling (see fig. a below);

After normalizing, the castings with complex or important shape shall be annealed to eliminate the internal stress.

If there is excessive free cementite in the original structure of cast iron, it must first be heated to the temperature of 50 ~ 100 ℃ of the upper limit of AC1, and then high-temperature graphitization is carried out to eliminate the free cementite (see Fig. b).

How to Heat Treat Gray Cast Iron? 6
How to Heat Treat Gray Cast Iron? 7

The effect of heating temperature on the hardness of cast iron after normalizing is shown in the following figure.

Within the normalizing temperature range, the higher the temperature, the higher the hardness.

Therefore, when the normalized cast iron is required to have high hardness and wear resistance, the upper limit of heating temperature can be selected.

How to Heat Treat Gray Cast Iron? 8

Fig. 8 Effect of normalizing temperature on hardness of gray cast iron

Note: the content of elements in the figure is mass fraction (%)

The cooling rate after normalizing affects the amount of ferrite precipitated and thus the hardness.

The larger the cooling rate, the smaller the amount of ferrite precipitated and the higher the hardness.

Therefore, the method of controlling the cooling speed (air cooling, air cooling, mist cooling) can be adopted to achieve the purpose of adjusting the hardness of cast iron.

3. Quenching and tempering

1. Quenching

The quenching process of cast iron is to heat the casting to the temperature of AC1 upper limit + 30 ~ 50 ℃, generally 850 ℃ ~ 900 ℃, so as to transform the structure into austenite, and keep it warm at this temperature to increase the solubility of carbon in austenite, and then quench it.

Oil quenching is usually used.

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The castings with complex shape or large size shall be heated slowly.

If necessary, they can be preheated at 500 ~ 650 ℃ to avoid cracking due to uneven heating.

The effect of quenching heating temperature on the hardness of cast iron is shown in table 8.1 below, and the chemical composition of cast iron listed in the above table is shown in table 8.2.

The higher the austenitizing temperature, the higher the hardness after quenching.

However, the higher the austenitizing temperature, not only increases the risk of deformation and cracking of cast iron, but also produces more retained austenite and reduces the hardness.

The effect of holding time on hardness is shown in Fig. 9 below.

Table 8.1 effect of austenitizing temperature on hardness of gray cast iron after fire (oil itching)

Gray cast iron

As cast

HBW

790°C

815C

845°C

870°C

A

217

159

269

450

477

B

255

207

450

514

601529

C

223

311

477

486

D

241

355208

469487

486520

460

E

235

512

F

235

370

477

480

465

Table 8.2 chemical composition (mass fraction) (%) of several cast irons

Cast ironTCCCSiPSMnCrNiMo
A3.190.691.700.2160.0970.760.030.013
B3.100.702.050.800.270.370.45
C3.200.581.760.1870.0540.640.005Trace0.48
D3.220.532.020.1140.0670.660.021.210.52
E3.210.602.240.1140.0710.670.500.060.52
F3.360.611.960.1580.0700.740.350.520.47
  • TC – total carbon content.
  • CC – the amount of bound carbon.
How to Heat Treat Gray Cast Iron? 9

Fig. 9 Effect of original structure of cast iron metal matrix on hardness after quenching at 840 ° C for different holding time

Chemical composition (mass fraction): 3.34% C; 2.22%Si; 0.7%Mn; 0.11%P; 0.1%S

The hardenability of gray cast iron is related to graphite size, shape, distribution, chemical composition and austenite grain size.

Graphite reduces the thermal conductivity of cast iron, thus reducing its hardenability.

The coarser and more graphite, the greater this effect.

2. Tempering

The effect of tempering temperature on the mechanical properties of cast iron is shown in Fig. 10 below.

In order to avoid graphitization, the tempering temperature shall generally be lower than 550 ℃, and the tempering holding time shall be calculated as t = [casting thickness (mm) / 25] + 1 (h).

How to Heat Treat Gray Cast Iron? 10

Fig. 10 effect of tempering temperature on hardness and strength of quenched cast iron

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