The influence of mold humidity and pouring temperature on occurrence of residual stresses in iron castings.

Budic, Ivan ; Novoselovic, Daniel ; Vitez, Ivan 等

Abstract: During process of formation of castings residual stresses occur. The paper establishes the present day state of knowledge on parameters influencing residual stresses in iron castings, based on writings on this subject. Furthermore, testing examined the influence of parameters mold humidity and pouring temperature on residual stresses. Tests have been carried out on standard grating test piece made of grey cast iron, poured in sand molds. The obtained measurements have been drawn in diagrams and the influence of parameter has been evaluated.

Key words: residual stresses in iron castings, mold humidity, pouring temperature.

1. INTRODUCTION

Production processes favor by and large creation of residual stresses (for example casting, welding, machining, heat treatment) by capturing residual stresses within the work piece. Any sort of machining leads to creation of residual stresses which results in reduction of tensile strength, occurrence of permanent distortions, increase of tensile corrosion, reduction of durability and frequently might lead also to breakage of the construction (Heidl, I. & Husnjak, M. , 1987)

Residual stresses occur during process of formation of the casting, especially from the moment of building of sufficiently stable crust until attainment of the room temperature.

2. THE INFLUENTIAL PARAMETERS

The residual stresses are frequent defect in production of castings, which is the reason that they are recurrently examined.

After completion of the cooling off period, the casting retains residual stresses, which equal the sum of stresses having different origin. If residual stresses surpass tensile strength, distortions and cracking of castings are to be expected in the process of manufacture or in stage of operation.

Some typical casting defects caused by great residual stresses are gray casting distortion due to unsymmetrical allocation of stresses, as consequence of unequal cooling off of the plate, and discloses casting cracks which occurred in warm or cold condition.

The survey of influential parameters on residual stresses is shown in description of single research procedures. The residual stresses in castings depend on numerous parameters, some of them acting simultaneously.

The occurrence of residual stresses in iron castings depend on the influence of parameters such as:

* Casting design

* Mold humidity

* Mold hardness

* Pouring temperature

* Chemical composition (CE)

* Cleaning of the castings

3. EXECUTION OF RESEARCH

To extend the scope of previous knowledge on residual stresses in iron castings, tests were conducted on standard grating poured out of grey cast iron in single sand mold having size 210 x 370 x 55/55 mm, figure 1.

[FIGURE 1 OMITTED]

Tests were conducted in accordance with the test schedule 24, which means that four influential parameters were examined on two levels.

The influential parameters during testing had the following range:

* Mold humidity, V = 4 and 8 %

* Mold hardness, Tv = 60 and 90 GF

* Pouring temperature, J = 1350 and 1400[degrees]C

* Chemical composition CE = 4,3 and 4,4 %

3.1 Calculation of residual stresses

Residual stresses were calculated according to the following equation:

[sigma] = [epsilon] x E (1)

[epsilon] = [[epsilon].sub.2] - [[epsilon].sub.1] (2)

Where:

[sigma]--Residual stresses, N/[mm.sup.2]

[epsilon]--Real distortion, [micro]m/m

E--Module of elasticity, in calculation taken to be 105 000 n/[mm.sup.2]

[[epsilon].sub.1]--Initial (Referential) distortion value. [micro]m/m

[[epsilon].sub.2]--Distortion of grating probe after removal of loading, [micro]m/m

Distortions were measured by application of electric resistant strain gages of the type 1,5/120 LY 11, by the manufacturer Hottinger Baldwin Technik GMBH. The strain gage is glued on the grating probe and connected by wires to digital device for measurement of distortion DMD 20 of the same company, figure 2.

[FIGURE 2 OMITTED]

Firstly, the initial (referential) distortion [[epsilon].sub.1] a is being measured on the grating probe containing residual stresses.

After that the outer bars of the grating probe are cut by hand saw. No increase of heat of the grating probe is allowed during process of cutting, which might exert influence on measuring results. Cutting of external bars leads to removal of residual stress of the grating probe.

After cutting of the bars distortion of the grating probe 2 a after removal of load is being measured. Finally, the real distortion [epsilon] is being measured according to (1).

4. TEST RESULTS

Test results have been shown in Table 1. On the basis of obtained values given in Table 1., diagrams of dependency of residual stresses on influential parameters have been draw. Also for each model (B1 ... B4) mathematical model has been calculated with regression analysis.

[sigma] = [a.sub.0] + [b.sub.1] x V + [b.sub.2] x [??] (3)

Mathematical model for Model B1:

[sigma] = -205,123 + 5,366 x V + 0,178 x [??] (4)

Mathematical model for Model B2:

[sigma] = -41,903 + 4,134 x V + 0,069 x [??] (5)

Mathematical model for Model B3:

[sigma] = 181,185 + 6,328 x V +,151 x [??] (6)

Mathematical model for Model B4:

[sigma] = 119,165 + 4,798 x V + 0,0117 x [??] (7)

Models (4) ... (7) show how the increase of mold humidity and pouring temperature leads to increase of residual stresses. This can be explained by the fact that higher mold humidity results in faster cooling and solidification of the grating prove, then with molds having lower humidity content. Also models shows that higher pouring temperatures lead to higher residual stresses which are confirmed in research.

5. CONCLUSION

This paper presents the influence of some parameters on residual stresses in castings on the basis of available referential data. Further it examines the influence of mold humidity and mold hardness on residual stresses occurring during casting of standard grating probe (test piece).

Test results confirmed by previous research. Further studies and statistical processing of data with more technologically influential parameters are to be expected, as well as publishing of obtained results on other kinds of castings.

6 . REFERENCES

Heidl, I.; Husnjak, M. (1987):Tenzometrija (Tenzometry), Tehnicka enciklopedija 12, Leksikografskog zavoda "Miroslav Krleza", 688-690

Table 1. Test results

Model V, % [??], [degrees]C [sigma], MPa Constant

B1 4 1350 56,28 CE = 4,3 %
 8 1350 78,74 Tv = 60 GF
 4 1400 66,18
 8 1400 86,65

B2 4 1350 67,42 CE = 4,3 %
 8 1350 84,41 Tv = 90 GF
 4 1400 71,32
 8 1400 87,40

B3 4 1350 47,30 CE = 4,4 %
 8 1350 73,96 Tv = 60 GF
 4 1400 56,20
 8 1400 80,16

B4 4 1350 58,30 CE = 4,4 %
 8 1350 77,92 Tv = 90 GF
 4 1400 64,60
 8 1400 83,36