The Effect of Boron in Ductile Iron
by Lyle R. Jenkins

When ductile iron is melted in a coreless induction furnace containing a silica lining with boron oxide to produce the crucible, the first iron used to sinter in the lining contains a boron pickup which causes low hardness in pearlitic ductile iron. Normal boron content of ductile base iron is 0.0015%. During sintering in the furnace lining, the boron climbs to about 0.0028%. This level reduces the effect of copper being used to produce pearlite and will continue to do so until the boron level drops below 0.002%. The first iron out of a newly lined furnace should be used for ferritic castings and the higher boron will not be noticed.

Since there is minimal data on the effect of boron on ductile iron it is helpful to consider the effect of boron on steels and also on malleable iron.

Boron is a strong carbide and nitride-forming element and increases strength in quenched and tempered low carbon steels through the formation of martensite and the precipitation strengthening of ferrite. Boron-containing killed carbon steels are available as low-cost replacements for the high-carbon and low-alloy steels used for sheet and strip. The low carbon boron containing steels have better cold-forming characteristics and can be heat treated to equivalent hardness and greater toughness for a wide variety of applications, such as tools, machine components, and fasteners. Boron is added to fully killed steel to improve hardenability. The amount added is a range of 0.0005 to 0.003%. It is most effective in low carbon steels. The effect of boron improving hardenability varies notably with the carbon content of the steel. The effect is much less in high carbon steels. The full effect of boron on hardenability is obtained only in fully deoxidized (aluminum-killed) steels. High-temperature treatment reduces the hardenability effect of boron. Only 0.001% boron is required for an optimum hardenability effect when appropriate protection of the boron is afforded by additions of titanium or zirconium. In carburizing steels, the effect of boron on case hardenability may be completely lost if nitrogen is abundant in the carburizing atmosphere.

Boron has no effect on the tempering characteristics of martensite, but a detrimental effect on toughness can result from the transformation to nonmartensitic products. For quenched and tempered steels, a practical way of improving toughness without reducing strength is to use a boron-containing grade of steel with a lower carbon content. The benefit of boron is applicable only to quenched and tempered steels: boron reduces the toughness of as-rolled, as-annealed, and as-normalized steels. Boron can cause hot shortness and can impair toughness. Boron has no effect on the strength of normal hot rolled steel but can considerably improve hardenability when transformation products such as acicular ferrite are desired in low-carbon hot-rolled plate.     back to top

Nickel-base superalloys show an improvement of creep properties by very small additions of boron and zirconium. It is believed that boron and zirconium segregate to grain boundaries because of their effects on carbide and gamma-prime distribution. Boron may also reduce carbide precipitation at grain boundaries by releasing carbon into the grains. The segregation of misfitting atoms to grain boundaries may reduce grain-boundary diffusion rates.

The addition of boron to malleable iron increases the number of nuclei available for the solid-state graphitization reaction. This can be achieved in two different ways, as follows:

1. By adding elements that increase undercooling during solidification. Typical elements in this category are magnesium, cerium, bismuth, and tellurium. Higher undercooling results in finer structure, which in turn means more gamma-Fe/3C interface. Because graphite nucleates at the gamma-Fe/3C interface, this means more nucleation sites for graphite. This also prevents the formation of unwanted eutectic graphite (mottle).

2. By adding nitride-forming elements to the melt. Typical elements in this category are aluminum, boron, titanium and zirconium.

Sources of boron may include:

• Wrought nickel-base alloys

• Cast cobalt-base superalloys

• Wrought iron-base alloys

• Boron treated steels

• Malleable iron

• Normal ductile iron

• Ductile produced during lining sinter

The boron pick-up experienced during lining sinter is only one furnace full and decreases with tap and charge back. The condition usually lasts about four hours under normal operating conditions.

The boron content is not lost in melting in cupola or induction furnace except by dilution. The addition of boron to the pouring ladle may result in carbides at lower level than when it is picked up in melting or during the sintering of a coreless induction furnace with a silica-boron oxide lining.

The method of chemical analysis for boron is faster by spectrometer, but care must be taken because the spectral lines of sulfur and boron are very close and sulfur can splash over into the boron and cause an error. The method for chemical analysis for boron is atomic-absorption. This is a little slower, but reliable. Another method for analysis for boron is a wet method, which requires boron-free glassware and takes about seven hours.

There is a need for research to determine if boron, causing soft castings, also causes reductions in toughness and fatigue strength. It should be determined if boron can improve nodule counts and reduce segregation in heavy section castings. It should also be determined if the use of boron can improve heat-treated ductile iron.

Because of the effect of boron on steel, it becomes necessary to determine similar or other effects on ductile iron. Until such time as an investigation can be undertaken, please be advised to watch carefully for the presence of boron in your castings and low hardness on pearlitic iron.        

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