The Role and Emerging Use of Niobium in Tool Steel

Summary

A variety of materials have been developed to perform as tools. However, tool steels are still the chief tool materials. Both the traditional grades, developed empirically, and new grades are being upgraded by special processing or by the use of new metallurgical concepts, e.g. microalloying, to optimize properties.

Niobium (Columbium) a carbide-forming element, is a newcomer to this field which is progressively finding its way into a variety of tool steels.

Following a review of the role of niobium in tool steels, some examples of actual applications and development work with niobium-alloyed tool steels will be highlighted.

Introduction

The first technological application of iron was for tools as one can see with findings from the early Iron Age (Hallstadt period 750 to 450 BC). Already at this time iron was produced by smelting processes and the heat treatment of hardening was applied. Caius Plinius Secundus (23 - 79 AC) describing iron in his book "Naturalis Historica" wrote: "This is both the best and the worst tool for mankind" emphasizing so the great variety of its application in daily life and in war.

Over the last 2500 years many new requirements for tools have arisen and therefore many different tool steels have been developed, most of them empirically. Actually the class of steels called tool steel comprises only about 1 volume % of steel production (1) but its significance is much bigger since almost all articles of our daily life are produced by tools.

Many tool steel grades are also used for other applications, and vice versa. The only fact in common is their use as tools; with regard to the variety of requirements for different applications, numerous compositions are known.

Usually tool steels rely on a variety of alloying elements to fulfill these requirements. Among them are the carbide-formers chromium, molybdenum, tungsten and vanadium. Therefore, at a first glance it is surprising that niobium, being their neighbor in the periodic table, is only little used in tool steel. The reason might be seen in the empirical development of most of the steel grades and the fact, that niobium is plentiful and inexpensively available only after pyrochlore reserves were developed in Brazil and Canada during the 1960's.

The Role of Niobium in Tool Steel

The basic consideration when applying niobium to steels, is that it forms very stable carbides to facilitate grain refinement and precipitation hardening as strengthening mechanisms in structural steels. Such an addition is usually in the range of 0.01 to 0.20 % and therefore called 'microalloying'. However, this effect of NbC is of minor importance in tool steel where quench-hardening is generally applied. To achieve reasonable martensitic hardness tool steels usually exhibit a medium to higher carbon content.

Figure 1 (2) shows the solubility product of niobium carbide in austenite for that carbon range. The solubility of NbC is very small, even at high hardening temperatures. At 11OOoC, growth of NbC precipitates is limited and significant particle coarsening does not occur - figure 2 (3). Hence the NbC particle will provide a similar grain boundary pinning effect as in the traditional normalized steels or in case hardening grades, where such "microalloy" additions are used to guarantee a fine grained microstructure. Figure 3 (4) shows, how these stable NbC particles control the austenite grain size: a high volume fraction of fine particles provides a smaller austenite grain size.

Such basic considerations make it clear that the predominant importance of microalloy niobium additions to tool steels is by preventing grain growth during reheating before hardening. The rather small amount of about 0.02% niobium, which might be dissolved during austenitization should add to slightly retarded transformation and also precipitation hardening during annealing. However, with the larger amounts chromium, molybdenum, vanadium, etc. present in many tool steels, the contribution of NbC is rather minimal compared with the effects of other alloy carbides.

The question arises, how to make use of this austenite grain refining effect of NbC particles. Two possibilities are apparent:

a) Besides strength/hardness, a certain "toughness" is required avoiding immediate brittle cracking of the tool due to local high stresses. Since brittle intercrystalline fracture can be related to carbide precipitates at the former austenite grain boundaries, figure 4 (5), a fine grain size would be very helpful to reduce the amount of carbides at the grain boundaries and so improve "toughness".

b) The high stability of NbC makes it possible to apply higher soaking temperatures without experiencing extreme grain growth. At higher soaking temperature, a larger amount of other alloy carbides will be dissolved resulting in overall better properties, e.g. higher red-hardness at low cost, and without impairing toughness.

These considerations show that the microalloy addition of niobium to tool steels provide an opportunity to redesign alloying and processing conditions of standard tool steels. A higher soaking temperature without impairing toughness will improve other properties for a constant alloy design; for equal properties the total alloy content could be leaner. Another economical benefit could be achieved by substituting long soaking times with slightly higher soaking temperatures for shorter times. Further grain refinement could be very helpful where high toughness is required. Such considerations on the role of austenite grain size have been made already elsewhere (6).

Due to the relatively large atom size of niobium compared to iron, niobium is prone to segregation. During solidification, niobium will be partitioned to interdendritic regions and a Fe+NbC eutectic might be formed. The volume percentage of this eutectic depends mainly on the niobium content of the steel, as shown in figure 5 (7). But also a higher carbon content or slower cooling rate (e.g. ingot casting) promotes the eutectic formation.

Since base steel composition and casting procedure are normally fixed parameters, limitation of niobium contents to less than 0.10% will provide the grain refining effect without the formation of conspicuous Fe-NbC eutectics.

Tools requiring high wear resistance are chiefly based on a ledeburitic microstructure with a high volume fraction of carbides. In figure 6, a schematic diagram using data of Miilders (8) is shown, indicating that toughness in these steels is naturally much lower than in alternative tool steels with a lower carbide content. In this figure high speed tool steels with coalesced carbides are in the upper range of the scatter band for the ledeburitic grades.

In ledeburitic steels, niobium carbide can be usefully applied. Compared to the chromium carbide M7C3 HV max) and the molybdenum-tungsten carbide M6C (1650 HV max), the cubic NbC exhibits a higher hardness (2400 HV). Compared to the hard cubic carbides of lighter metals, TiC or VC, it has the benefit that its density of 7.8 g/cm3 is in the range of the specific weight of liquid tool steel.

Pure NbC is somewhat heavier than liquid steel, which can be advantageous in promoting wear resistance e.g. to the outer surface of centrifugally cast parts or to the bottom side of ordinary castings. However, in high alloyed steels, the density of NbC is influenced by alloying with other alloy elements. For instance, vanadium dissolves into primary niobium carbide lowering its density.

Inspection of the iron corner of the ternary Fe-Nb-C-diagram figure 7 (9) indicates that with only small additions of niobium, NbC will be precipitated in liquid steel. This diagram has been recently confirmed with several steel compositions by dilatometry (10).

However, in the alloying process of adding niobium to high carbon iron and steel, standard ferroniobium Fe-65%NS does not readily dissolve into liquid solution (11). Figure 8 demonstrates that the dissolution of ferroniobium results from a carburizing process with the formation of several layers on the surface of the lump. These carbides are subsequently released into the melt, probably via melting of the eutectic Fe-NbC.

The kinetics of this process can be optimized by stirring, adding NbC or FeNbC instead of FeNb, or by using a master alloy with a eutectic matrix. Ser alto: injection of tens in gray cabtron

The latter has been demonstrated by adding 8%Nb as Fe-80%Nb to a simple melt, of composition Fe-5Cr-1V-2.2C, without stirring. As figure 9 shows, the eutectic matrix of Fe-80%Nb melts (eutectic temperature around 1,450oC) and breaks the lump down.

Since large carbides will impair toughness (12), other procedures can be used to produce finer particles when toughness is an important requirement, for example inoculation with Al203, TiN etc., as well as by powder metallurgy routes (faster cooling rate in powder production), or by reducing the carbon potential in the liquid state and subsequent carburizing/nitriding in the solid state (13).

The positive role of high niobium contents to improve wear resistance is shown in the diagram prepared from data developed by Locker (14) - figure 10. These results achieved with high speed tool steel are confirmed with 12% Cr ledeburitic irons. These steels usually exhibit around 15 Vol. % of ledeburitic M7C3 carbides (15). With the addition of niobium-forming MC carbides the wear rate is very much improved especially against materials which exhibit a higher hardness than the M7C3 carbides, e.g. Al203 (2000 HV) or SiC (2600 HV) - figure 11. Due to a more favorable carbide distribution, the advantageous effect of MC carbides is more pronounced in the as cast condition.

Examples of Industrial Production

As already mentioned the great variety of tool application demands a wide range of steel compositions. A systematic classification will not be attempted in this paper, but the examples described will follow approximately a sequence of increasing alloy content. However, these data of commercially-produced, niobium-containing tool steels only represent a small selection of possible tool steel compositions which could benefit from niobium additions.

For a long period, plain carbon steel was the most important tool steel for the various hard tools, simply produced by hardening in water.

The example in figure 12 is one of the first where niobium has been used in tool steel. It is a plain carbon steel with about 0.60%C. Niobium has been added to achieve the required hardness. Initially, the added niobium content was about 0.20%Nb, but presently it has been lowered to about 0.04% which achieves the same purpose but reduces the possibility of the formation of Fe-NbC eutectic (16).

Low alloy steels such as AISI 4140 (42 CrMo 4) are also a standard product for several hand tools etc. Chaparral Steel, in the US, has introduced a new product which achieves equal properties after direct quenching from the forging temperature (17). This steel, Microtuff-10, is cost-effective owing to its overall leaner alloy design (about 0.12%C, 1.80%Mn, 0.18%Mo,0.10%Nb) and by avoiding further hardening and tempering.

Standard plastic mould steel typically exhibits a chemical composition of 0.40%C, 1.50%Mn, 2.0%Cr and 0.20%Mo. Since good machinability and photoetching ability is required, besides the mechanical properties at working temperature, well defined sulphur addition plus sulphide shape control together with a steel-making process which minimizes macro- and micro-segregation is common. In the event that more chemically aggressive plastics are processed, the chromium content is increased up to and above 13%.

For this rapidly increasing market (the production of plastics is actually about 100 Mio t/a) new requirements also arise. In this context, a new steel grade has been developed which exhibits a higher wear resistance. In addition, this steel grade is also weldable (18). Its typical composition is as follows: 0.12%C, 2.0%Mn,0.35%Cr, 3.5%Ni, 1.2%Cu, 1.10%Al, 0.10%S and 0.06%Nb

The steel achieves the required properties by age hardening heat treatment. Figure 13 shows that the highest hardness, of about 44 HRC, is obtained by solution annealing at 900 to 925oC followed by ageing for 6 hours at 500oC. Typical applications are in injection molding of camera or electronic parts, or in compression molding dies such as the dies for plastic containers shown in figure 14.

Alloy design considerations to optimize H13 tool steels has been made elsewhere (19), indicating that with the substitution of 0.08%Nb for 0.50%V, better overall properties are achieved due to more effective grain size control.

This work has been continued by more detailed experimental investigation (20) and full scale trials with a 2 tonnes ESR heat of 0. 37%C, 5.35%Cr, 1. 25%Mo, 0.35%V and 0.07%Nb. The results confirm that up to 1020oC no grain growth is observed and further heating does not result in a higher hardness. Therefore 1020oC hardening followed by 540oC tempering is recommended for excellent strength and outstanding toughness properties.

Several tools have been fabricated. Figure 15 shows examples of dies for aluminum castings, i.e. for an office punching machine and for a fan baffle. These tools achieved a slightly better performance of about 25% to 35% more castings over their lifetime than traditional H13, which contains about 1%V.

In processing heavy non-ferrous metals such as copper, somewhat higher alloyed hot work die steels are common. Among this group of steels, Bohler W335 is microalloyed with niobium. This steel is especially applied for extrusion and pressure casting dies, swages etc. Figure 16 shows one example of its successful application. The steel exhibits a chemical composition of 0.38%C, 1.70%Mn, 2.60%Cr, 2.60%Mo, 0.75%V, 0.006%B and max.0.12%Nb(21).Due to the niobium addition, a higher hardening temperature of up to 1080oC can be applied, guaranteeing excellent hot strength even at temperatures above 600oC as shown in figure 17.

For the extrusion of automobile valves, Carpenter Technology (22) reported that "Thermowear" (0.58%C, 4%Cr, 2.5%Mo, 1%V, 3%Co, 1.5%Nb) achieved up to 100% better lifetime compared to the even more highly alloyed AISI H19. Niobium is added to "Thermowear" to form wear-resistant carbides. Moreover, higher hardening temperature of up to 11OOoC could be applied without excessive grain coarsening.

Warm forging is gaining acceptance over quenched and tempered steels due to advantages of dimensional stability, oxidation resistance, near net-shaping and elimination of subsequent heat treatment. "VTM" is a steel developed by Aços Villares (Brazil) (23) to meet the strength, toughness and wear-resistance required by warm forging tools. This tool steel containing 0.58%C, 4.5%Cr, 2.7%Mo, 1.8%W, 0.8%V, 0.45%Nb,features an impact resistance four to five times better than M2 with an equivalent tempering hardness of 57-62 HRC. Niobium forms undissolved carbides with a suitable morphology to contribute wear-resistance without impairing toughness.

The chief characteristic of this group of steels is, that they should not experience higher than 200oC working temperature. Therefore secondary hardening potential is of no priority. The steel is designed for maximum martensite hardness (0.80 to 1.00%C) and if this is not sufficient, hard particles (i.e. carbides) are introduced in the martensitic microstructure. For each application, an optimum balance between high wear- resistance and low toughness has to be found. For heavier tools, alloying with elements which improve hardenability, and so allow softer (i.e. oil) quenching, offer a reasonable compromise.

Rolls for cold rolling mills are often manufactured from medium alloyed steels, e.g. about 0.8%C, 2%Cr, 0.5%Mo, 0.2%V, to achieve both a high surface hardness and a reasonable hardening depth. Microalloying this steel with about 0.10%Nb guarantees the existence of stable carbides at a high solution treatment temperature, i.e. up to 10000C. Hence, hardenability is improved (figure 18) but the amount of retained austenite, a major source of breakdowns owing to spalling, is reduced. These microalloyed cold rolls have a performance at least 20%. better compared to the non-microalloyed rolls (24) and have become an established product.

The most common alloying element to increase the volume fraction of carbides is chromium. Besides a 5% chromium-steel (1:2363 = 1.00%C, 5.3%Cr, 1.1%Mo,0.2%V) a ledeburitic chromium-steel (1:2379= 1.55%C,12.0%Cr,0.7%Mo,1.0%V)finds widespread application for clipping punches, shear blades and other dies. A recent development brought out a new cold work tool steel, Bohler K340 = 1,1%C,8.3%Cr, 2.1%Mo,0.5%V, 1% Al and 0.10%Nb(25). Owing to its finer carbide distribution this steel exhibits improved toughness reducing tool failure. Besides the almost tripled impact toughness, this steel also has a somewhat higher hardness than the 12%Cr - steel, with better softening resistance, which, therefore, also allows surface treatments like nitriding, figure 19.Good wear-resistance, electrical discharge machinability and low roughness guarantee excellent behavior in a variety of uses, especially in shear blades and stamping tools; figure 20 gives an example.

Another example is a martensitic chromium steel for cutting tissue, figure 21. Traditionally high chromium ledeburitic steels have been used to provide

For many years, a 13% Cr-steel alloyed with molybdenum and vanadium, 1.4153, has been used successfully, and a further optimization has been possible by introducing niobium to this steel. Figure 22 (26) shows that a 25% better toughness is obtained for a certain hardness level, by combining a fine and homogeneous distribution of primary niobium carbides with a secondary precipitation of vanadium carbides. The higher molybdenum content of the optimized steel adds to the corrosion resistance. Altogether the fatigue strength is improved by 50%, which made this steel very successful in this application.

The resistance of ledeburitic chromium steels can be remarkably improved by adding niobium to form primary cubic carbides, see figure 11. There are numerous applications, e.g. in hammer crushers for corn mills or in molds for the ceramic industry, which contain niobium with additions as high as 5%.

Hard facing electrodes for 'build-ups' on earth moving components, crushers, cement mills etc. are also ledeburitic steels. These filler wire electrodes may exhibit very high alloy contents of approximately 4%C,25%Cr,7%Mo,7% Nb plus V, W etc. guaranteeing a surface hardness up to 67 HRC. This is one of the most traditional fields of niobium application in tools. Besides the usage of ferroniobium powder some producers successfully utilize ferroniobium carbide (27) in electrode manufacture.

High Speed Tool Steels

This group of steels combines the high wear-resistance of cold work die steels with the secondary hardening effect of hot work die steels. Most of these grades also have been empirically developed. There are tungsten, molybdenum, and tungsten + molybdenum-alloyed grades. The most important one is the M2 (S6-5-2) with 6%W, 5%Mo and 2%V. For improved red hardness, cobalt additions as high as 5% or 10% are applied.

Recently a new HSS, Bohler S620(28), has been especially developed to avoid the addition of cobalt. This steel with a base composition of 1.1%C, 4.3%Cr, 6.4%W, 5%Mo, 1.9%V also contains 1.1%Al and is microalloyed with 0.07%Nb. It is mainly used to machine nickel-base alloys or titanium-alloys, especially for interrupted machining. This steel shows better performance than the higher alloyed grades M42(S2-10-1-8) or T42(S1O-4-3-10) which contain 9-10%Co, thus offering a great cost advantage. Figure 23 shows typical examples where this new high speed tool steel is applied.

Various approaches have been pursued to introduce larger amounts of niobium- forming, primary carbides into high speed tool steel. Aços Villares, substituted the 3%V in grade T42 (S10-4-3-10) by 2.2% Nb + 0.5%V finding better performance as a cutting tool. This steel VILLARES VK-10 N (29) is regularly produced for more than a decade.

The matrix concept has been used to design a steel where NbC has substituted for all the primary carbides which exist in an M2 steel (30). This lean-alloyed steel exhibited a base composition of about 1%C,4%Cr,3%W,3% Mo,1.2%V and 2.6%Nb.

Microanalysis confirmed that all the niobium existed in angular carbides, typically less than 20 microns, figure 24, but these carbides were considered to be rather large for several applications.

Compared to standard M2 steel, which is overall more highly alloyed, this experimental steel showed a somewhat lower hardness (about 1.5 HRC lower) but excellent toughness measured in the bending test (31). This combination of excellent toughness with the large hard carbides has been found to yield an improved performance for drilling soft materials, such as austenitic stainless steel. The lifetime of such drills has been tripled compared to drills of M2, but in drilling quenched and tempered steels the standard M2, with its higher carbide volume fraction and hardness, was superior.

Niobium-alloyed HSS has been object of a variety of experimental investigations (e.g.32-34). The solidification behavior, the microstructure and the properties/ performance of niobium-alloyed HSS produced by various routes have been extensively studied in Leoben/Austria (10,11,14). The results of these laboratory experiments suggest that in cast ingots, a small particle size of the niobium-carbide can be assured with niobium-content less than 1.5%. Higher niobium contents may need further process control.

Tirgoviste Works, the biggest tool steel producer in Romania, has extended these results into industrial production, by designing and making a steel with an optimized matrix composition, niobium- alloying and titanium inoculation. Their standard production route of t heats and 1 t ingots has been maintained to produce this steel containing 1.1%C, 4.4%Cr; 4.6%Mo,2.0%W,1.6%V and 0.7%Nb. An excellent homogeneous carbide distribution was observed, thanks to the inoculation of primary niobium carbide with titanium nitride (figure 25).

A hardness of 65 HRC has been obtained by triple tempering after hardening and the tools produced (screw taps, reamers, drills) showed about 15% better performance than the traditionally-used higher alloyed M2. Within comparative investigation of tool wear with various HSS grades including powder steels, this metallurgically-optimized alloy composition resulted in the lowest flank wear level of all grades (10).

Current trends in development are focusing on processing technology, such as different powder routes, laser-alloying etc. (35) and on alloy design to allow higher NbC additions to be made without extreme particle growth (36).

Other Steels and Alloys Applied for Tools

Finally two examples are mentioned, where niobium containing materials are applied for tools:

The martensitic stainless steel AISI 630 corresponds to 1.4548(X5CrNiCuNb 17 4 4) and finds its predominant use in aircraft, rocket and mechanical application. Compared with other corrosion resistant steels, this steel offers the benefit of uniform hardness distribution and best flatness due to the fact that final precipitation hardening is used as a strengthening mechanism.

A typical composition of Bohler grade N700/702 is: 0.03%C, 0.75%Mn, 16.5%Cr, 4.4%Ni,4.1%Cu and 0.40%Nb, where niobium is used as a stabilizing element. The high hardness of 45 to 50 HRC is achieved by precipitation hardening at 480oC after the solution treatment. It allows final surface machining in the precipitation hardened condition (37).

By using electroslag remelting and a plate cross rolling technology this steel has been found to be the optimum solution for press plates used for the production of laminates based on phenolic, epoxy or polyester resins, as well as copper-clad laminates for printed circuits, hardboard and chipboard, plasterboard, fibre cement boards etc. Figure 26 shows one example of press plates for multilayer production.

Oxidation resistance and stress rupture capability at elevated temperatures is needed for many applications such as aircraft engine turbine blades. One of the most successfully applied materials is the Fe-Ni-base Alloy This alloy contains niobium, which forms the coherent phase 8 Gama "and is also present in the Gama' phase and therefore is one of the major strengtheners of this alloy.

A typical chemical composition of IN718(= 2.4668) is: 0.04%C, 19%Cr, 3.1%Mo, 52.5%Ni, 0.6%Al, 0.09%Ti,0.005%B, 5.3%Nb, remainder Fe.

This alloy is also successfully applied as a tool material, e.g. for drop forging blocks in presses working steel (38). These tools usually have a longer contact with the heated workpiece than in hammer forging, therefore the standard hot work die steels are often not suitable for this application.

Summary and Conclusion

Tool steels have been developed empirically over many centuries and are often tailored for a specific application. Improved knowledge of Physical Metallurgy is helping to optimize alloy and processing design, especially when new requirements for processing advanced materials arise. In this context niobium plays an interesting role since:

a) Microalloy additions of niobium guarantees grain size control even at comparable high hardening temperatures. This makes many other alloying elements more effective with regard to their influence on hardenability without causing any negative effect on toughness.

b) Primary niobium carbides improve the wear resistance significantly and this can be successfully utilized in high speed tool steel or ledeburitic chromium steels, when niobium is added at the several percent level. These primary carbides provide wear resistance alongside other traditional alloy carbides but niobium carbide appears to be more effective due to having a higher hardness and favorable ratio Me/C ratio.

Despite the fact that niobium is plentiful and commercially available only in the last thirty years, several tool steels apply already niobium microalloy or alloy addition. It can be expected that in the future other tool steels will follow this promising approach.

Acknowledgements

Within the report reference was made to several companies, which provided information and photos. Special thanks are due to the following colleagues: Ivan Falleiros (Aços Villares, Brazil), Edmund Haberling Thyssen Edelstahl AG, Germany), Karl Leban Bohler Ges.m.b.H.,Austria), Wilfried Noll (Stihl, Germany) and Siegfried Wilmes (Bohler AG, Germany), who supplied relevant information for this report, and to Prof. S. R. Keown, who revised the original text.

Special thanks are due to M.S. Andrade and J. R. T. Branco (Cetec-Brazil) and B. Gonzalez (UFMG-Brazil) for their assistance in several investigations.

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Write to the Authors

K. Hulka

  Niobium Products Company GmbH - 49 (211) 138 010

J. R. C. Guimarães

  CBMM - 55 (11) 828 8855

(*) This paper was first published in February 1993.