Gray Cast Iron Performance – Mechanical Properties

The mechanical property data of gray cast iron is generally obtained from separately cast ⌀30mm test bars. Since the microstructure and mechanical properties of gray cast iron are significantly affected by the cooling rate during the solidification and eutectoid transformation zones, the properties measured from these test bars do not fully represent the performance of actual castings with different shapes and wall thicknesses. For castings with special requirements, test bars with cooling rates similar to the critical sections of the castings should be selected, either as separately cast or attached test bars, to determine performance.

 

Tensile Strength

Tensile strength is the main indicator for evaluating gray cast iron performance. Countries classify grades of gray cast iron based on tensile strength. Gray iron parts subject to tensile and bending loads must have their tensile stresses calculated, using safety factors between 2 ~ 12.

Based on a large amount of test data, some empirical formulas have been derived and applied in numerical simulation and thermal analysis systems.

For example, the tensile strength (MPa) of ⌀30mm test bars with a pearlitic matrix can be calculated as follows:

R_m = 786.5 − 150×w(C)% − 47×w(Si)% + 45×w(Mn)% + 219×w(S)%

Where w(C)%, w(Si)%, w(Mn)%, and w(S)% represent the mass fractions of carbon, silicon, manganese, and sulfur in the test bar, respectively. 

The tensile strength (MPa) at a particular section of a casting can also be estimated from its chemical composition and hardness using this empirical formula:

R_m件 = 258.3 + 1.275HBW − 67.3×w(C)% − 25×w(Si)% − 31×w(P)%

The difference between calculated and actual values is within ±21 MPa. The cooling rate is reflected through the hardness at that section.

 

Elongation After Fracture

Gray cast iron has very low elongation after fracture. For grades from HT150 ~ HT300, elongation at fracture ranges between 0.3% ~ 0.8%. Elongation increases with higher tensile strength but decreases with higher silicon content. The permanent deformation during fracture is between 0.2% ~ 0.6%.

There is a certain correlation between elongation and fatigue limit. Structural designers have proposed requirements for gray cast iron with tensile strength over 300 MPa and elongation over 1%.

 

Compressive Strength

Gray cast iron components used as machine bases or load-bearing structures require compressive strength calculations. Its compressive strength is very high, generally 3 ~ 4 times that of steel.

Grades of Gray Cast Iron in the United States	Tensile Strength/MPa	Compressive Strength/MPa
20	152	572
25	179	669
30	214	752
35	252	855
40	293	965
50	362	1130
60	431	1293

When testing compressive strength, the length-to-diameter ratio of the sample is 2:1. If it is 1:1, the measured compressive strength is 10% ~ 12% higher.

There is a certain relationship between compressive strength (MPa) and Brinell hardness (HBW): For non-inoculated gray cast iron: ratio is 3.4 ~ 4.0; For inoculated gray cast iron: if hardness < 175 HBW, ratio < 3.7; if > 175 HBW, ratio > 3.7

Unlike ductile materials like steel or malleable cast iron, gray cast iron exhibits almost no plastic deformation before failure under compressive load. At low stress levels, the compressive elastic modulus is 3% ~ 5% higher than the tensile modulus.