What Is The Distinction Between Ductile Iron And Grey Iron?

Mar. 29, 2022

Continuous casting of ductile and grey iron controls the industry in regards to low product expense, high machinability, and also high performance. Both metals are utilized for a wide range of applications, consisting of building and construction equipment, pipe fittings, oil area machinery as well as also transport solutions.

Ductile Iron Pump Volute

Each metal has its own benefit because of the differences in carbon levels, as well as being utilized for various applications according to their toughness.
Among the essential determinations that have to be made pertaining to the production of your components relies upon the distinction between ductile iron and grey iron. The characteristic differences between the two make them optimal for some objectives as well as particularly excluded for various other functions. The differences are directly connected to the reality that ductile iron is really an alloy including mainly iron as well as a variety of various other metals in much smaller-sized quantities. Magnesium is one of the most frequently utilized components, with cerium and tellurium used less typically. Numerous quantities of nickel, copper, or chromium have likewise been utilized to change approximately 30% of the iron to enhance corrosion resistance. Copper, as well as tin, have likewise been used to improve tensile strength.
The main distinctions between the two can be broken down into 5 crucial areas. The very first two belong in that they are measurements of how the material can be adjusted. The ductility of a metal is a dimension of the capacity of a metal to flaw or lengthen while under stress. Tensile stamina is similar yet is a measurement of the capacity of a metal to hold up against compression. Ductile iron executes much better in these two groups, largely because it is designed with that said objective in mind. As a result of the composition of this alloy, it withstands flaking particularly grey iron. Ductile iron likewise has minimum tensile stamina that goes to the optimal common tensile array for grey iron.
The third area of difference remains in impact resistance. Ductile iron carries out better around, too. As a basic policy, ductile iron can endure 10-15 foot-pounds of influence while grey iron can just withstand up to 2 foot-pounds of impact.
The 4th comparison is the thermal conductivity of the two products. According to Ductile.org, "Product buildings which contribute to excellent thermal tiredness resistance are high thermal conductivity, reduced modulus of elasticity, and the high toughness and also ductility." They conclude that grey iron is superior to ductile iron because of this thermal tiredness resistance.
The fifth and last particular comparison is the ability of the product to moisten vibration. In this field, grey iron once more carries out much better than its cousin. The flaking characteristic that works against grey iron in ductility permits it to take in and distribute resonances throughout its entire surface area.

Key Differences

  • Ductility – Ductility is determined by a greater percentage of elongation under tension.  The addition of magnesium in ductile iron means that the graphite has a nodular/spherical shape conferring higher strength and ductility as opposed to gray iron which is flake shaped.  For example, 18% elongation can be easily achieved with ASTM A395 and A536 grades 60-40-18 material.

  • Tensile and Yield Strength – There are certainly differences when it comes to tensile and yield strength of gray and ductile iron.  Ductile iron has a minimum tensile strength of 60,000 psi and a minimum yield strength of 40,000 psi.  There are many grades of gray iron as per the ASTM A48 standard.  While gray iron does not have a measurable yield strength, the range for tensile strength is 20,000 psi – 60,000 psi.

  • Impact – Impact strength, also known as toughness, is a measure of a metal’s ability to resist fracturing while absorbing and impact (collision). Ductile iron has a greater resistance to impacts, and is able to resist a minimum of 7 foot pounds of impact (versus 2 pounds of impact for gray iron). This means that while ductile iron can be used in critical applications that involve impact, gray iron has limits that prohibit it from being used for certain purposes.

  • Thermal Conductivity – Ductile iron has a lower thermal conductivity than gray iron. Specifically, the graphite phase in gray iron gives it very high thermal conductivity as the heat transfer is through the graphite flakes. The isolated spheres of graphite in ductile iron lower its thermal conductivity considerably – in fact it is not much more conductive than steel. Is the amount of carbon increases, the thermal conductivity increases. This means that lower strength gray irons will have higher thermal conductivity. The design engineer needs to keep this in mind if the primary reason for choosing gray iron is for thermal conductivity. Gray iron, with its high thermal conductivity, is a great choice for parts that undergo thermal shock, like brake drums.

  • Vibration Damping – Internal friction is how materials absorb vibrational energy. Gray iron dampens vibrations more effectively than ductile iron because gray iron exhibits non-elastic behavior at very low stresses. A higher damping capacity enhances fatigue resistance because the time duration when stress is at or above the fatigue limit is reduced. This enhancement of fatigue resistance is the most beneficial aspect of higher damping capacity. The other major benefit is the reduction of vibrations and noise that can be emitted by components of machinery.

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