Boron Element Boron
Atomic number 5
Density, 20° C (68° F) 2.34 g/cm3
Atomic weight 10.81
Melting point 2079° C (3774° F)
Boiling point (sublimes) 2550° C (4620° F)

GENERAL

Boron is added to steels for its unique ability to increase hardenability when present in concentrations of around 0.0015% to 0.0030%. It has long been used as a replacement for other alloying elements in heat treatable steels, especially when these constituents were in short supply. Boron is also used in austenitic stainless steels to control hot shortness, or to improve their creep properties. Boron is added to some steels for the nuclear industry, where neutron absorption is required. It is also used in deep drawing steels, where it removes interstitial nitrogen and allows lower hot rolling temperatures.

Boron is also a key component in FeNdB magnets, which are among the most powerful permanent magnets known.

Boron reacts readily with oxygen and nitrogen and is, unfortunately, completely useless in steel when in combined forms. Great care must therefore be taken during steelmaking to ensure that boron is adequately protected. Failure to recognize this requirement can lead to erratic heat treatment response.


AVAILABLE FORMS

Boron is supplied to steelmakers as ferroboron or as one of several proprietary alloys. Choice of addition depends, as always, on steelmaking practice, product mix and volume, individual operators' experience and preference, and price. A steelmaker should choose that addition agent giving the highest and most reliable recovery consistent with his overall melt shop economics.

Ferroboron is the lowest cost addition agent. Boron content is relatively high: standard grades are sold with incremental boron levels between 12 and 24% B. Major impurities are carbon (0.10-1.5%), silicon (0.30-4.0%) and aluminum (0.5-8.0%). A typical analysis will include 18.0% B, 0.50% C, 0.50% Si, 0.2% Al, 0.03% P, 0.01% S. All except boron are maximum values. Product is supplied in lump form, 2 in. or 1 in. x down, packaged in 250 kg or 500 lb steel drums, or supersacks (bulk bags) of up to 3000 lb (1360 kg) capacity. Many customers apply a minimum size limit, such as 5mm (0.2 in.), in order to minimize the amount of fine material, which can give poor recoveries in less well-controlled melting practices. Ferroboron is also available as cored wire.

Because ferroboron does not contain appreciable concentrations of protective elements, it requires greater care than the proprietary alloys in order to give adequate and consistent results. It is normally added after other oxygen/nitrogen scavengers, such as ferrotitanium.

The proprietary boron addition agents are more expensive than ferroboron on an initial cost basis but are often preferred for their greater efficiency, ease of application and more consistent results. All will contain varying proportions of oxygen and/or nitrogen scavengers such as titanium, aluminum, silicon and zirconium. These elements generally have an even greater affinity for oxygen and nitrogen than does boron.

The most common proprietary addition agent typically contains 2.0% B, 15% Al, 30% Ti, 10% Si, bal. Fe. This product's high scavenger/boron ratio ensures its effectiveness for all boron steels, provided they have been adequately deoxidized first.

A variety of other composition proprietary boron addition alloys are available, with boron contents varying between 0.5% and 4%. Generally, the higher the ratio of boron to scavenger elements, the greater the care required to ensure adequate recovery of the boron in the steel.

Proprietary boron addition agents are sold in lump form 1-1/4 in. and 2 in. x down, packaged in bags, cans or large drums.


ADDITION PRACTICE

Boron combines aggressively with oxygen and nitrogen dissolved in steel; great care must be taken in steelmaking and addition practices, to prevent these reactions from occurring or the boron's effectiveness will be irretrievably lost. Steels should be fully killed before boron is added: aluminum killing provides additional protection against nitrogen but steelmakers with continuous casters prefer to keep aluminum levels as low as possible to forestall nozzle blockage problems. Vacuum carbon deoxidation or AOD processing, where possible, reduce oxygen levels without the need for large scavenger additions.

Standard practice is to add boron to the ladle after all other alloying additions have been made and between the time the ladle is 1/4 to 3/4 full. Precautions against reoxidation of the heat through the use of inert gas shrouded nozzles, synthetic slags, etc., are highly recommended. Boron 'fade' (loss of effectiveness in the last alloy cast) can be prevented by using slightly higher aluminum and/or titanium contents.

Mold additions of boron were made successfully for many years, but general experience has been that these give less uniform results than carefully controlled ladle additions.

Cored wire additions can be made at the ladle furnace, ladle stir station or degasser.


ROLLING/FORGING

In their zeal to make sure enough boron went into solution, steelmakers used to add much more than the minute amounts required. The result was a hot-short steel that would break up in the roughing stands or during hot forging. Even if successfully hot worked, such steel often had poor room temperature impact properties.

It is now recognized that these problems stem from the formation of a low melting point B-C-Fe eutectic (Fe
2B/Fe3C/Fe), which forms when the boron content exceeds about 0.007%. Only soluble boron is effective for hardenability, so the normal aim composition is 0.0015% to 0.0030% B. Boron does segregate, and dangerously high concentrations can form even when average boron contents are within specification.

Other than that, boron steels will roll and forge just about the same as their plain carbon or alloy counterparts. There is a slightly greater danger of overheating, though. Boron's diffusivity in steel is about the same as carbon's, and it is possible to deboronize steels in high temperature oxidizing atmospheres. Likewise, furnace atmospheres should be kept low in nitrogen to prevent the formation of boron nitrides.

The scale formed on boron steels is not so tightly adhering as that on other alloy steels; this property is claimed to improve die life.


HEAT TREATMENT

Boron suppresses the nucleation (but not growth) of proeutectoid ferrite on austenitic grain boundaries. Various theories have been advanced to explain this: most propose that the presence of boron on or near these boundaries reduces strain- or interfacial energies, so lowering the driving force for ferrite nucleation. The result, however, is that the TTT curve for a steel containing as little as 0.0005% B will be markedly shifted to the right compared to a similar, but boron-free, carbon or alloy steel.

Boron's effectiveness increases linearly up to about 0.002% B, then levels off. Upper limits are set by the hot working and embrittlement considerations described above. Most specifications for boron steels now define the allowable limits as 0.0005-0.007%.

Carbon content has a strong effect on boron's hardenability factor, F
B where

  hardenability of (base steel + boron)
FB  =   
  hardenability of base steel

The empirical formula given is F
B = 1 ± 1.5(0.9 - %C). Thus, boron will be much more effective at low carbon levels, its contribution falling to zero as the eutectoid carbon content is approached. An interesting sidelight on this phenomenon is that carburized boron steels are marked by a high core hardenability but a low case hardenability.

Boron is unique among alloying elements in that its hardenability factor increases with the amount of martensite chosen as the standard. Boron will have a relatively greater effect on a section that can be fully hardened than on one which quenches only to, say, 50% martensite. As a practical matter, the effect of boron on hardenability is often denoted by the 'Boron Factor', the ratio of the ideal diameters of a boron steel to the same steel with no boron. Medium carbon steels should have boron factors in the range 2.0-2.5. Lower values indicate a loss of boron effectiveness through poor steelmaking practice or improper heat treatment.

Boron has no significant effect on either the Ae, or Ae
3 temperatures. The Ar1 is similarly unaffected although the Ar3 is lowered somewhat. Thus, boron steels should be heat treated the same as comparable boron-free steels. Overheating should be avoided. Boron is a grain coarsener and hardenability will not improve with increasing grain size; exactly the opposite is true here. However, hardenability lost through overheating can be regained by slow cooling and reheating to the proper quench temperature. Oxidizing and nitriding atmospheres should be avoided; boron steels should not be carbonitrided.

Boron neither raises nor lowers the MS temperature and has no effect on retained austenite. It will not change the fineness of pearlite, nor will it produce any solid solution strengthening in ferrite. It shows no effect on tempering response except a slight but tolerable increase in susceptibility to temper embrittlement. The magnitude of this susceptibility is such that, if a boron steel is used to replace a molybdenum grade, the danger of embrittlement will be greater; if a Cr-Mn steel is replaced, it will be reduced.

APPLICATIONS

Carbon-manganese-boron steels are generally specified as replacements for alloy steels for reasons of cost: C-Mn-B steels are far less expensive than alloy steels of equivalent hardenability. Applications for these steels include earth scraper segments, track links, rollers, drive sprockets, axle components and crankshafts.

Boron alloy steels are specified when the base composition meets mechanical property requirements (toughness, wear resistance, etc.), but hardenability is insufficient for the intended section size. Rather than call for a more highly alloyed (therefore more expensive) steel, a user may simply specify the corresponding boron grade, thereby ensuring suitable hardenability.

An expanding area of boron usage is the field of high strength low alloy and other structural steels. These may be supplied as hot rolled or as quenched and tempered (for boron grades, the latter are more common). Boron assures adequate hardenability in heavier plate sections.

Boron is sometimes used in non-heat treated steels. Ferroboron may be added as an intentional nitrogen scavenger in carbon steels for automotive strip stock. By avoiding interstitial nitrogen, boron makes the steel more formable. Aluminum is sometimes used for a similar duty, but AlN is slower to precipitate, so requiring higher annealing temperatures. Boron addition makes the steel more formable and eliminates the need for strain age suppressing anneals.

Boron has a high neutron absorption capability. For this reason, it is added to certain types of stainless steel for use in the nuclear industry. Levels of 4% boron or more have been used, but the lack of hot ductility and weldability mean that boron contents of 0.5 to 1.0% are more common for neutron absorption application. Nonetheless, even at these boron contents, the ferroboron has to be of the highest purity.

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