Boron in Steel: Part Two
Boron is useful as an alloying element in many materials, but in this paper it will be illustrated as an alloying element in the steel because of its effect on hardenability enhancement. Boron is added to unalloyed and low alloyed steels to enhance the hardness level through enhancement hardenability. Boron added to high-speed-cut steels, for example, containing 18%W, 4%Cr and 1%V, enhances their cutting performance, but reduces their forging qualities.
Boron in steel as an alloying element
Boron is useful as an alloying element in many materials, but here it will be illustrated as an alloying element in the steel because of its effect on hardenability enhancement. Boron is added to unalloyed and low alloyed steels to enhance the hardness level through enhancement hardenability. Boron added to high-speed-cut steels, for example, containing 18%W, 4%Cr and 1%V, enhances their cutting performance, but reduces their forging qualities.
Addition of boron in a quantity of up to 0.01% to austenitic steels also improves their high-temperature strength. Boron steels are used as high-quality, heat-treatable constructional steels, steels for carburization and cold forming steels such as steels for screws. The addition of 5 to 50 ppm B to ferritic steels containing 14 to 18% Cr may improve the surface quality of stainless strips by avoiding errors, such as scale, ribbing a roping, and ridging, which otherwise frequently occur in strip production.
The basic effect of boron on in the steel is the enhancement of hardenability, which is evident already at a very small concentration, of the degree of 0.0010% of boron. It is added to unalloyed and low alloyed steels for the hardness level enhancement through the hardenability. Even in the small quantity of the degree of size up to 100 ppm, boron gives the same effect of the hardenability enhancement as other more expensive elements which must be added in much bigger quantity. For example the addition of 30 ppm B in SAE replaces approximately 1%Ni, 0,5%C, 0,2%Mn, 0,12%V, 0,3%Mo or 0,4%Cr.
The Figure 1 shows hardenability curves of the boron low alloyed steels (13MnCrB5) compared to the steel without boron (16MnCr5).
An addition of 30 ppm of boron in steel which contains approximately 0.15%C, 1%Mn and 0.9%Cr shows a clear increase in hardness of almost 50% to a larger depth from the surface than in the case of a steel of identical composition, but free from boron (see Figure 1). According the same authors, there is no difference in hardness on the surface between the boron-containing and the boron-free steel, which can be seen in the Figure 1, too. Accordingly, the incipient hardness is therefore determined not by boron, but by the martensitic structural state influenced by the carbon content. The hardness-enhancing effect of boron comes into play only below the surface.
The mechanism which is decisive for the enhancement of hardenability by boron is a delay in the transformation to the bainite, ferrite and pearlite structures, which are softer than martensite. Unless prevented by boron, these softer structures would be formed during the cooling from the austenitisation temperature, after annealing or hot working.
Figure 1: Boron effected hardenability of steel
The Boron hardenability effect
An outstanding feature of boron steels is the improvement in hardenability produced by the addition of even a minute quantity of boron. It is generally accepted that a hardenability peak is reached when the quantity of boron is between 3 and 15 ppm. In an excessive amount of boron (>30 ppm) is present, the boron constituents become segregated in the austenite grain boundaries, which not only lowers hardenability, but also may decrease toughness, cause embrittlement and produce hot shortness. The affect of boron on hardenability also depends on the amount of carbon in the steel. The effect of boron increases in inverse proportion to the percentage of carbon present.
Boron must be in its atomic state to improve hardenability, which means that care must be taken during steel production for the boron to be effective. Boron may also become ineffective if its state is changed by incorrect heat treatment. For example, high austenitizing temperatures must be avoided as well as temperature ranges where certain boron precipitates occur.
Hardenability is highly dependent on the behavior of oxygen, carbon and nitrogen present in steel. Boron reacts with oxygen to form boron oxide (B2O3); with carbon to form iron boroncementite (Fe3(CB)) and iron boroncarbide (Fe23(CB)6); and with nitrogen to form boron nitride (BN). Loss of boron by oxygen is presented by making the boron addition to silicon-aluminum killed steels and by using good ladle and mold practices. Strong nitride formers (titanium, aluminum, and zirconium) protect the boron from reaction with nitrogen. For example, if nitrogen is fixed by using titanium, satisfactory hardenability is obtained in the temperature range up to 1830°F (1000°C) provided that the steel contains about 5-20 ppm of boron.
The hardenability of boron steel is also closely related to austenitizing conditions and is generally said to decrease by heating above 1830°F (1000°C). Boron steel must also be tempered at a lower temperature than other alloy element steels of the same hardenability.
Applications
Boron steels are used for a variety of applications, as a wear material and as a high strength structural steel. Examples include punching tools, spades, and knives, saw blades, safety beams in vehicles etc.
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 and 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 (HSLA) 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.
This range of medium carbon steels with a deliberate boron addition improves hardness during heat treatment (Boron 922, C=0.25-0.30% and Boron 921, C=0.38-0.42%). Toughened by tempering following oil or water quenching, boron steels possess a hardness equivalent to that of much higher carbon steels and of more expensive low alloy steels. Advantages of boron steels are; improved cold formability, lower delivered hardness giving improved blanking tool life, improved weldability due to low carbon equivalents, lower tempering temperatures giving savings on energy, and good case hardening response. Typical applications include toecaps and chains.
The type of boron steel used on vehicles today has extremely high strength. The boron steel used on Volvo cars has a yield point of about 1,350-1,400 MPa (196,000-203,000 psi). That’s about four times stronger than average high-strength steel. But the process used to make it that strong takes away some of the steel’s workability properties, such as being able to straighten it.
For now, boron steel is found primarily on European vehicles, such as the dash panel on the 2002 Porsche Cayenne SUV, the safety bar around the rear seats on the 2003 Porsche Boxster, the door guard beams on the 2003 Porsche 911 Carrera, and the inner B-pillars on the 2003 Mercedes-Benz E Class. Volvo probably uses boron steel the most. Boron is used on the bumper reinforcements and guard beams on the 2004 Volvo S40 sedan and 2005 V50 station wagon. The 2003 Volvo XC90 SUV has several applications of boron steel, including the inner B-pillar reinforcements, the roof bow between the B-pillars (if there is no sunroof), and the inner rear body panels. The 1999-2004 S80 and 2001-2004 V70 and S60 also have boron steel inner and outer rear body panels.