Universal High Strength (UHS) Steel

Now days, most of the aircraft/aerospace, automotive and gas/oil equipment is made of expensive, high strength and high toughness steel, which is considered to reflect good quality of the product and its endurance. Yet, all the enhanced properties of high-quality steel are typically achieved through the expensive processing (vacuuming or double vacuuming and re-melting to reduce sulfur content) and use of substantial concentrations of Cobalt (Co) and Nickel (Ni).  Both, in fact, are expensive and rare elements.

The additional manufacturing processes, combined with the cost of material, have driven the price of such alloys to between 6 and 10 times higher than ordinary steel, thus increasing the cost of this steel by as much as $8 per pound or $17,600 per metric ton.   At this price level, these products are only suitable for the most exclusive applications in the industries such as defense, aircraft and aerospace.  Yet, even the defense industry, which is in great need of high quality steel, often cannot afford the product because of its price.

In turn, ISAI’s Universal High Strength (UHS) Steel is the solution for many industries which require high strength steel for a reasonable price. The United States Patent and Trademark Office (PTO) awarded ISAI a patent (8,137,483 B2) for a martensitic steel that exhibits high strength and moderate toughness.  Our steel is a replacement for AF1410 or Aeromet 100 at a substantially lower cost of the material applied.

Over a seven-month period, three variations of the UHS steel were produced which produced savings in the millions of dollars in development costs.   UHS220 is a low cost replacement for AerMet 100 and Marage 250 steels.  It is cobalt-free with less than 11.5% of the weight being alloying elements.

In contrast, the alloying elements in AerMet 100 represent 28.8% of the weight, while the total weight for elements in Marage 250 is as much as 32% of the steel’s weight.  UHS230 is a low cost replacement for AISI 4340 and 300M.   UHS230 is cobalt-molybdenum free with less than 1% of the weight in nickel and less than 7% of the weight of all alloying elements.  UHS240 is the least expensive of the three articles.   It is also a cobalt, molybdenum, and nickel free product, which reflects strength and impact toughness even higher than AISI 8640 steel.   UHS240 finds it’s uses in applications where high strength and toughness as well as wear resistance are of prime importance.

Mechanical Properties

The ultimate mechanical properties of our UHS steels depend on the concentration of the alloying elements and the heat treatments applied.   The steel can vary in a wide range of substances, thus satisfying the most demanding clients through different variations of the alloys employed in the product.

Mechanical Properties

UHS 220

UHS 230

UHS 240

Rockwell Hardness Scale

55 282 263

Ultimate Tensile Strength (ksi)

54 297 12

Yield Strength (ksi)

57 304 9

Charpy V-notch Impact Energy (ft-lb)

26-36 18-32 18-27

Fracture Toughness K1c (ksi ×√in)

80-110 60-100 60-70

Table 1: Mechanical Properties

Table 1 provides a list of mechanical properties of three articles of the UHS steels. UHS 220, UHS 230 and UHS 240 are the basic products introduced in market. Besides, as mentioned above, technical characteristics can be changed and adjusted according to customer’s need.

Chemical Composition

The composition of the introduced steels consists of carbon (C); ferrite stabilizing chromium (Cr), aluminum (Al), silicon (Si); strong carbide forming vanadium (V), titanium (Ti), niobium (Nb); austenite stabilizing nickel (Ni), manganese (Mn), copper (Cu); iron (Fe) and other impurities.

  • Carbon (C) Carbon (C) contents of 0.3% to 0.45% wt. for the UHS 220, UHS 230, and UHS 240 support  the formation of carbides, at least of an  element selected  from the group consisting of vanadium (V), titanium (Ti), niobium (Nb) or complex  carbides  as centers  of growth of martensite laths  fanning  the  microstructure of tempered dispersed lath martensite with retained  austenite, where contents of retained  austenite  are less  than 25% for UHS 220, less than  15% for UHS 230, and less  than  10% for UHS 240.
  • Chromium (Cr) When being added to the steel, chromium increases its strength, hardenability and temper resistance. UHS 220, UHS 230, and UHS 240, all contain chromium (Cr), which is being added at most 2.5% wt.
  • Molybdenum (Mo) Molybdenum (Mo) is used efficiently for improving hardenability, reducing reversible temper brittleness, resisting hydrogen attack and sulfide stress cracking, increasing elevated temperature strength. The molybdenum is contained only in UHS 220, with up to 1 % wt., thus being eliminated from UHS 230 and UHS 240 for the more efficient cost.
  • Nickel (Ni) Nickel (Ni), an austenite stabilizing element, supplies high toughness, however its concentration is limited in the martensitic structure.  The content of Ni is included at most 3.5% wt. in UHS 220, at most 1.0% wt. in UHS 230, and is completely eliminated from UHS 240.
  • Manganese (Mn) Manganese (Mn) increases hardenability and tensile strength of steel, but to a lesser extent than carbon. It is also able to decrease the critical cooling rate during hardening, thus increasing the steels hardenability much more efficient than any other alloying elements.
  • Silicon (Si) Silicon (Si) strengthens the steel matrix by increasing the bonds between atoms in a solid solution.  It protects the grain boundary from the growth of carbides and, thus, decreases the toughness of the new steel. The concentration of Si varies from 0.1 to 1.3% wt. in all UHS 220, UHS 230, and UHS 240 steels.
  • Copper (Cu) Copper (Cu) plays a great role in enhancing mechanical properties such as corrosion resistance, ductility, and machinability. The preferred amounts of Cu vary from 0.1 to 1.0% wt. but the concentration of Cu should be less than the concentration of Si (see Table 2).
  • Aluminum (Al) Aluminum (Al) plays an important role in deoxidizing of the steel. It is added to all UHS 220, UHS 230 and UHS 240 with 0.25% wt.
  • Iron (Fe), phosphorus (P), sulfur (S) The balance of the steel is iron (Fe) and incidental impurities. Small amounts of phosphorus (P), sulfur (S) and other incidental elements will not critically affect the mechanical properties of the new steel.
  • Vanadium (V), niobium (Nb) and titanium (Ti)  Vanadium (V) affects the structure and properties of the new steel in several ways. Firstly, it forms finely dispersed particles of carbides in austenite. They, in turn, control the size and shape of grains, by precipitating vanadium based, fine dispersed secondary carbides during tempering. Therefore, it also positively affects the kinetic structure and morphology of the austenite-martensite transformation.

 

In contrast to vanadium (V), small concentrations of a strong carbide-forming niobium (Nb) do not affect the kinetics of phase transformations. Yet, the content of vanadium is essential for the steel as it helps to inhibit austenite grain growth during the heating.

While being a similar element to vanadium, titanium (Ti), however, is a stronger element in the formation of active carbides, and is also a vital element for the steel production.

For this reason, at least one element selected from a group consisting of V, Ti, and Nb is a part of the new steel.  The concentrations of (V+Ti+Nb) vary from 0.1 to 1.0% wt.

Alloying Element

UHS 220 % weight

UHS 230 % weight

UHS 240 % weight

Carbon (C)

0.3 to 0.45 0.3 to 0.45 0.3 to 0.45

Chromium (Cr)

at most 2.5 at most 2.5 at most 2.5

Molybdenum (Mo)

at most 1.0

Nickel (Ni)

at most 3.5 at most 1.0

Manganese (Mn)

0.3 to 1.0 0.3 to 1.5 0.3 to 3.5

Silicon (Si)

0.1 to 1.3 0.1 to 1.3 0.1 to 1.3

Copper (Cu)

0.1 to 1.0 less than Si 0.1 to 1.0 less than Si 0.1 to 1.0 less than Si

Vanadium+Niobium+Titanium (V+Ti+Nb)

0.1 to 1.0 0.1 to 1.0 0.1 to 1.0

Aluminum (Al)

at most 0.25 at most 0.25 at most 0.25

Sum of Alloying Elements

less than 11.5 less than 7.0 less than 6

Iron Ore (Fe)

remainder remainder remainder

Table 2: Chemical Composition


Processing

Based on the optimum alloying concentration, the critical temperatures and the contents of retained austenite lab scale ingots, the two discovered steels were melted and processed. Similarly, articles of new steels were heat treated and tested. The lab scale ingots were produced in an open air 100 lbs. induction furnace and casted into cylindrical graphite molds. When being poured into the forms, the metal reached 2950° F to 3000° F. In the next stage, the substance was exposed to a usual room temperature of 59° F to 86° F, followed by a homogenized annealing process of 60 lbs. of ingots heated at 2100° F to 2150° F for 6 hours.

Thereafter, ingots were rolled to a final size of approximately 1.5″ thickness plates and 1″ diameter rods. Last but not least, plates and rods were subjected to re-crystallization, annealing at 1100° F to 1150° F for 6 hours. Standard AS TM specimens for tensile and Charpy V-notch impact toughness tests were machined. The machined specimens were subjected to the following heat treatments: austenitization at 1580° F to 1650° F for 60 minutes, oil quenching for 2 to 2.5 minutes, and then air cooling to the room temperature. Some specimens were subjected to cooling at -120° F and tempering at 340° F to 450° F for 3 to 3.5 hours. After heat treatment, the specimens were subjected to mechanical tests.

Microstructure

Metallographic examination of the test alloys subjected to quenching and low-temperature tempering displayed a microstructure of tempered dispersed lath martensite, primary vanadium carbides, and retained austenite. The retained austenite content depended mostly on the content of nickel and to a lesser extent on the content of carbon.  Figure 1 displays the microstructures of laboratory-produced UHS, which was also a subject to quenching, refrigeration, and low-temperature tempering.  Earlier microstructural examination via transmission electron microscopy (TEM) detailed fine martensite laths with layer thickness of 10 to 15 Pm.

In low-temperature tempered martensite, stresses resulted in the formation of “cloud-like” clusters with a high density of dislocations on the order of 1011 per square centimeter. In addition to this structure, the martensite was partially twinned.  Vanadium carbide (primary), which did not dissolve at quenching temperatures (1625oC), precipitated during cooling after rolling (or forging).  During medium and high-temperature tempering, precipitation of other vanadium, chromium, and complex (Fe, Me)3C carbides was observed. The carbides precipitated primarily inside the martensite grains, and the carbide size was less than 180 Angstroms.

All of these factors contributed to the high hardness of 54 to 55 HRC and yield strength of 220 to 260 ksi. Microstructures of the studied alloys subjected to quenching and low-temperature tempering consist of finely dispersed tempered lath martensite, carbides of vanadium, and retained austenite. The retained austenite content depends on the nickel and carbon contents, and on the quenching process. Analysis of Charpy sample fracture surfaces reveals quasi-cleavage fracture with varying amounts of ductile dimple rupture component, as shown in Figure 2.

Production Cost

True production cost of alloys is difficult to assess, as it depends on the cost structure of a particular producer. However, based on the published prices of alloying materials and on experience with several steel producers, we estimate that the production cost of the UHS 240 can be a martially available SAE/AISI 4340 grade. Comparing UHS 240 to AerMet 100, the cost of the charge material alone is approximately eight to ten times less in the today’s market.

Type of Steel

Cost of Material USO/Mt

AerMet 100

$18,020

Marage 250

$16,650

UHS220

$ 2,800

AISI 8640

$ 1,850

300M

$ 1,760

Table 3:Cost Comparison of High Strength High Toughness Steels

Benefits

While UHS is capable of replacing a number of high strength industrial steels, its reduced complexity also increases production volumes, decreases investment costs, the number of melts and the inventories of steel producers. The cost effective compositions are achieved by selecting the ratios between the austenite stabilizing, ferrite stabilizing, and carbide forming elements, selecting processing procedures and heat treatment. All this contributes to the main benefit of UHS which is a reduction in cost is reached without a reduction in mechanical properties.  Thus, we believe that our Universal High Strength Steel is a unique innovation that will make a positive change in the modern world of high strength high toughness steel production.