HOT TOPICS Issue # 5, 2003

Effect of Boron in Ductile Iron

1) Basic elemental information Element & Atomic Atomic Melting Symbol Number Weight Point Boron (B) 5 10.82 3720 F 2) How Boron is introduced in the iron Sources of boron include tool steels, interstitial-free steels, hardenable steels, enameled cast irons and steels, malleable iron, and certain furnace lining materials. It has been reported that some silicon carbide sources can contain boron. Boron is not reduced or eliminated in normal melting practice. Recovery is considered to be >90 % from most sources when oxygen contents are low. As an alloy addition in malleable iron, boron combines with N to form boron nitride which increases temper carbon (nodule) counts and reduces annealing time. In steels, boron is added at levels of 0.001 to 0.003% to enhance hardenability. Boron readily reacts with O and N in steel, and to be an effective alloy addition, other oxide and nitride formers such as Al, Ti and Zr are added to protect the boron. A major source of boron can be furnace lining materials. Boric acid and boric oxide are common components in the binders. Boric oxide lowers the fusion temperatures, accelerates sintering and improves toughness. In coreless melting units, it has been said that boron is picked up in the first 4 hours of melting and that boron will increase from a nominal 0.0015% to 0.0028%. Subsequent heats are not susceptible to B pickup. However, in pressure pour furnaces, it has been reported that boron pickup persists much longer. One report indicated an increase from 0.0006 to 0.0061% after holding for two weeks. A different lining material resulted in raising B from 0.0009% to 0.0017%. In both instances the linings had been installed for more than six months. In another study where various alumina linings were investigated, boron pickup over a 6-hour period ranged from 0 to 0.0295%.

Boron is a stable element and does not burn off in re-melting. Therefore, it can accumulate and is only reduced by dilution over time. Segregation of suspect material might be warranted. 3) Effects on the Microstructure Boron increases the propensity to form ferrite and carbides in ductile iron. Levels above 0.0005% (5 ppm) can influence the ability to develop fully pearlitic microstructures in ductile iron. Levels above 0.002% will form stable carbides in ductile iron. As little as 0.008%B is known to cause filamentary carbides in the cell boundaries of ductile iron. The normal additions of Mn, Cu and Sn must be increased to counteract the ferritizing tendency of boron in pearlitic ductile iron. Various reports have been given on the influence of boron. One report stated that when B exceeded 0.0008% in pearlitic ductile iron, raising the Cu level from 0.23% to 0.47% was sufficient, but that additions of up to 0.16%Mn were ineffective. In the same study, additions of N actually lowered hardness and at the same time raised nodule count. In another trial with 93 to 111 ppm boron, the additions lowered the pearlite content from 80% to 45% in iron alloyed with 0.50%Cu and 0.0046%Sn. An addition of 0.41%Cu almost restored the pearlite, and a subsequent addition of 0.053%Sn fully restored the pearlitic microstructure. The investigators also noted pearlite content increased when the inoculation addition was cut by 75%. Of course the nodule count was also reduced. Doubling the inoculant addition further reduced the pearlite content. Boron additions have also been investigated in ferritic ductile iron. An iron with 20% pearlite and containing 0.20%Cu and 0.0006%B was unaffected by an addition of 0.0057% boron, thus indicating that B will not likely be effective in promoting ferritic structures in ductile iron.

4) Effects on the Mechanical Properties In general, addition of boron results in decreased tensile strength, yield strength and hardness in pearlitic ductile iron. In one study the mechanical properties of a 16mm (5/8 in) thick section was reduced by 38 N/mm² (5500 psi) and elongation was reduced by 5% with the inclusion of 0.01% B. The reason for the reduction was reported to be the precipitation of boro-carbides in the cell boundaries. In another study, a plant noticed a significant reduction in Brinell hardness of pearlitic ductile iron after holding the iron in pressure pour furnaces for several weeks. They studied the chemistry and found that the boron ranged from 0.0019% to 0.0059 % during the problem period. As the boron content dropped below 0.0017 % the hardness became normal again (200 - 210 HB) without changes in the other chemical residuals. In addition, as the B dropped below 0.0006 % the hardness increased to over 230 HB. In a third study, boron levels of less than 0.0010 % had little effect on ductile iron properties, whereas levels of 0.0100 % reduced tensile strength and hardness slightly and reduced elongation by >40 %. The material with the elevated B level displayed a cell boundary network of boro-carbides, which at high magnification looked very much like phosphide eutectic. Annealing Boro-Carbides In that study, annealing at 1750 F for eight hours failed to decompose the carbide network, but did inhibit the decomposition of pearlite during subsequent furnace cooling and holding at 1275 F. Analysis of Boron in Ductile Iron The fastest method for the analysis of boron is by optical emission spectrometry (OES), but care must be taken because the lines for B and S are very close and S can splash over and cause an error. The best methods for analysis are atomic absorption or a wet method using boron free glassware, which takes about seven hours. The boro-carbides are not readily dissolved in acid and misleading low results from chemical analysis may be encountered. Special techniques are required for wet chemistry of the element. 5) Environmental considerations

There are no known environmental or health concerns with boron as a dissolved component in cast iron. 6) Effects on melting and chill Melting losses are negligible and boron is not lost on re-melting. Boron recovery is considered to be >90%. It is known to cause carbide in the cell boundaries, but it is not known for causing chill at the normal levels encountered in ductile iron. 7) Considerations in various ductile iron grades a) In pearlitic ductile irons, boron should be held below 0.001%. Additions of Cu and Sn can reverse the ferrite-forming tendency of boron. The melting of pearlitic ductile should be delayed, rather than immediately following the relining of the furnace. b) In ferritic ductile iron, boron appears to have no effect. c) In hardenable ductile irons, boron is not known to impart added hardenability. 8) Effect of section thickness No reports on the effects of boron in various section sizes were found. 9) Counteracting detrimental effects Copper and tin additions were found to counteract the ferrite forming tendencies of boron. Limited studies with manganese additions indicated that Mn was ineffective in restoring pearlite. Foundries should consider scheduling the melting of pearlitic iron for a number of shifts after relining the melting and holding furnaces to avoid soft castings. Because melting losses are very low, boron can accumulate in the iron. There are several sources of boron and it is only reduced by dilution over time. Segregation of suspect material might be warranted. 10) References “Trace Elements in Cast Irons”, R. Naro and J.F. Wallace, AFS Transactions, 1969. “The effect of Boron in Ductile Iron”, L. Jenkins, Ductile Iron News, 1995. “Boron Contamination in Ductile Iron”, R.D. Schelleng, Modern Casting, 1967.

Prepared by Richard Gundlach Climax Research Services, Inc