Understanding the Mechanical and Physical Properties of Iron
MECHANICAL PROPERTIES OF IRON
Mechanical Properties at Elevated Temperatures
The tensile strength of pearlitic ductile irons decreases continuously with increasing temperature. At 400°C, it’s approximately 2/3 of the room temperature resistance. For ferritic irons, the decline is less pronounced, with resistance at 400°C around 80% of room temperature. The yield strength (σ0.2%) for both ferritic and pearlitic irons remains stable up to 350-400°C, then decreases rapidly. Hot hardness is maintained up to 400°C, diminishing thereafter.
For temperatures up to 300°C, allowable stress in static structures can be based on room temperature σ0.2% values. Above 300°C, calculations should be based on creep data. Adding a small amount of molybdenum significantly improves creep and hot tensile strength in both ferritic and pearlitic irons.
Tensile Properties at Low Temperatures
Impact properties exhibit a temperature threshold below which tensile elongation decreases. σ0.2% increases with decreasing temperature, while tensile strength undergoes a transition. Above this transition temperature, tensile strength remains constant or increases. Below it, tensile strength decreases. Phosphorus and silicon increase the transition temperature and reduce ductility.
Tensile Strength
Tensile strength is the maximum tensile stress a material can withstand, dependent on the deformation rate.
Compressive Strength
Compressive strength is the maximum stress a material can withstand under crushing load. For materials failing due to fracture, it’s a well-defined property. For materials that don’t fracture, it’s the effort required to deform the material a specific amount. It’s calculated by dividing the maximum load by the original cross-sectional area.
Elongation
Elongation is a type of deformation, representing the change in length under tensile stress.
Torsional Stiffness
Torsional stiffness refers to the resistance to twisting or bending. In mechanics, it’s the change in shape under various forces, specifically the deformation caused by opposing forces acting in parallel planes, causing rotation between sections.
Resistance to the Court
Resistance to the court implies that the machine tool’s power exceeds the power needed to cut the workpiece material.
PHYSICAL PROPERTIES
Coefficient of Thermal Expansion
The expansion characteristics of cast irons are complex due to transformations in solution, graphite precipitation, graphitization, and austenite formation above 700°C.
Corrosion Resistance
Ductile iron often exhibits corrosion resistance similar to gray iron and superior to steel. Ductile iron pipes used in sewage applications can be protected by anodizing, zinc coating, plastic covers, or polyurethane coating.
Torque
Torque is characterized geometrically by the twisting of a curve parallel to the shaft axis around the shaft itself.
Fatigue of Materials
Fatigue refers to material failure under cyclic dynamic loads at stresses below the static fracture strength. An example is a wire repeatedly flexed until it breaks.
Cast Iron
Cast iron with interconnected graphite flakes in eutectic cells. Inoculation creates smaller eutectic cells for improved endurance.
Low tensile strength is due to large graphite flakes. Higher strength can be achieved by reducing the carbon equivalent through alloying or heat treatment.
Properties include high compressive strength, thermal fatigue resistance, and vibration damping.
Cast White
White cast irons are used for their hardness and abrasion resistance. Martensite can form during heat treatment.
Malleable Iron
Malleable iron is created by heat-treating white cast iron. Two types exist: whiteheart malleable iron (cooled slowly for graphitization) and blackheart malleable iron (cooled rapidly to form martensite).
DUCTILE CAST IRON (OR NODULAR CAST IRON)
Steps involved in creating ductile cast iron:
- Desulfurization: Sulfur is removed in the smelting furnace to prevent flake graphite formation.
- Nodulation: Magnesium is added to remove remaining sulfur and oxygen.
- Inoculation: An effective stabilizer is added after nodulation to prevent white iron formation.
Compact Graphite Iron
This type has a graphite form between flake and spheroidal, offering a balance of strength, ductility, thermal conductivity, and vibration absorption.
MALLEABLE CAST IRON
Produced by annealing white cast iron at 850-950°C to decompose cementite and form nodules of free carbon (temper carbon). Two types exist:
- European Malleable Iron: Annealed for 1-2 days in the presence of ferric oxide.
- American Malleable Iron: Annealed for up to 8 days, resulting in a ferritic structure.
NODULAR CAST IRON (OR DUCTILE CAST IRON)
Characterized by spheroidal or nodular graphite. Carbon content is similar to gray cast iron. Spheroidal graphite forms during solidification due to the presence of magnesium and cerium.
IRON
Iron refers to a broad group of ferrous metals that solidify with a characteristic structure. Fracture surface color helps identify the alloy. White cast iron has a white fracture surface due to carbides. Gray cast iron has a gray fracture surface due to graphite flakes deflecting cracks.