For the sake of convenience, the invention will be described in relation to security bars but it is to be understood that the invention may apply to material in other forms such as plate or sheet material. Such security bars may have various cross-sections such as circular, rectangular, square or polygonal.

BACKGROUND ART Security bars for jails, armories and other locations of unauthorised egress and ingress have, for many years, been supplied as a welded assembly of steel bars known in Australia as COMSTEEL P8.

COMSTEEL P8 is an austenitic steel grade with 1.1 to 1.25% carbon and 11 to 13% manganese. In castings, this material is frequently used in operational conditions where resistance to wear by impact is required such as in the mining and earth moving industries. In bar form, COMSTEEL P8 is inherently low in hardness but brittle in the remainder of its mechanical properties.

The main reason for using COMSTEEL P8 in security applications is its propensity to work-harden, that is, during an attempt to saw, drill, mill or turn, the austenitic crystal structure (Face-Centred-Cubic) will twin along the miller (111) plane, forming a new crystal structure with each grain which resists the free movement of dislocations throughout the grains. Thus, the more the

material is worked, the more the perception of resistance to machining increases.

However, a well trained operator can beat work-hardening by decreasing the load and speed exerted during the machining. Thus, a welded assembly of COMSTEEL P8 bars is not tamper proof since in the case of a jail security system, an inmate can overcome work-hardening by slow and light saw strokes as has been well documente in the past.

Another disadvantage of COMSTEEL P8 is that it does not lend itself to workmanlike welding because of the strongly cored and large austenite grains.

It has been observed that P8 bar assembly welds have cracked during the initial welding of the security screens. Egress from a welded COMSTEEL P8 assembly is readily achieved by a few blows with a heavy object causing the austenite grains in the heat affected zone adjacent to the weld to fail in a sudden and brittle mode.

Furthermore, welded COMSTEEL P8 assemblies have a pronounced propensity for stress-corrosion cracking during manufacturing which is exacerbated after installation and the use of impactive forces.

COMSTEEL P8 bars are no longer procurable in Australia and thus there is a need for a security bar which possess the positive benefits of the COMSTEEL P8 bar but in addition can provide a security grill which is tamper proof against attempts employing generic hand tools and which may be assemble by welding and which will respond to impactive forces like a spring and not deform or fail in a brittle mode.

SUMMARY OF THE INVENTION According to one aspect of the invention there is provided a hardened steel bar, grille, sheet, plate or other shaped material which is suitable for security purposes and which is formed from a nitridable steel which is

subjected to austenitization followed by oil quenching and then tempered and nitrided to provide a hardened steel material having a surface hardness of at least 50 HR.

According to another aspect of the invention there is provided a method of preparing the hardened steel material comprising the steps of:- (i) selecting a nitridable steel; (ii) subjecting the nitridable steel to austenitization, quenching and tempering to provide a heat treated material, and (iii) nitriding the heat treated material to provide a hardened steel material having a surface hardness of at least about 50 HRc.

The depth of nitriding may be from about 0.4 to about 0.6mm.

The surface hardness of at least about 60 HRC may extend to a depth of about 0.2mm and the hardness at the 0.4 to 0.6mm extent of nitriding may be about 30 HIC.

The nitridable steel preferably contains aluminium and may contain chromium and molybdenum. The aluminium content may be in the range of about 0.2 to about 2.0%, but preferably in the range of about 0.5 to about 2.0% or about 0.8 to about 1.2%.

The chromium content may be in the range of about 1.00 to about 1.80%. The molybdenum content may be from about 0.15 to about 0.40%.

The carbon content of the steel is preferably from about 0.28 to about 0.45%.

The nitridable steel may be AISI 4140 or any other steel from the AISI 4000 and 8000 series or B S En 41 B.

The family of nitridable steel grades is preferably limited to but a few and is generally restricted to the AISI 4000 and 8000-series mentioned above.

Suitable alternatives to AISI 4140 are AISI 4130N, AISI 4340, AISI 8135/40 and

AISI 8135/45. Insofar as plate is concerned, BISALLOY 500 and K10 are typical commercially available nitridable steel plates.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph of hardness (HRc) against depth of nitriding for a hardened steel bar according to an embodiment of the invention and for a galvanise hardened steel bar according to an embodiment of the invention.

MODES OF PERFORMING THE INVENTION Two sample hardened steel bars were prepared from En 41 B type steel (which contains aluminium). Austenitization was carried out by heating the bars to about 880°C and holding them at that temperature for about one hour.

Quenching was in a medium fast oil (12 to 14 cubic inches/second) in a Seal- Quench furnace. Tempering was carried out at about 320°C for about 1.5 hours in an air-recirculation furnace. Nitriding was carried out using the BRISTUFF (Trade Mark) process described below. One bar was galvanised.

The test bars were sectioned transversely such that a cross-section was produced. Vickers hardness testing (0.5Kg) was carried out on the sections, working inwards from the surface at 0.1 mm intervals. The Vickers values were converted to HRC (Hardness Rockwell"C") and graphed as shown in Fig. 1.

A first significant feature of the test results is that the galvanising process had negligible effect on the hardness profile. The reason for this would be that the galvanising temperature did not exceed the original tempering temperature of the steel.

The second significant feature of the results is that the hardening process resulted in two distinct regions. The first region is the surface region which exhibited a hardness of 60-67 HRc to a depth of 0.2mm. In the second region, the hardness drops rapidly down to 33 HRc at a depth of 0.5 to 0.6mm.

The En 41 B test material (which contains aluminium) is a preferred steel type which will achieve the greatest depth of hardening and the highest level of hardness at the surface. Other materials such as 4140 and 4340 will go close to achieving similar depth of hardening but the surface hardness will be less.

The difference is the surface hardness for En 41 B versus 4140 would be of the order of 65 HRc at 0.2mm as against 54 HRc at 0.1mm.

The heat treatment procedures prior to nitriding are similar for each nitridable steel as the parameters governing the balance of mechanical properties of the substrate are the Time-Temperature-Transformation, or TTT diagrams, which are essentially equivalent within the lower cost nitridable steel grades.

Preferably, austenitization is carried out by heating the material to about 880°C and holding it at that temperature for about 1 hour. The quenching step may be carried in medium fast oil (12 to 14 cubic inches/second) in a Seal- Quench furnace. The tempering step may be carried out at about 320°C for about 1.5 hours in an air-recirculation furnace.

The nitriding step may be carried out by using any convenient process such as the BRISTUFF (Trade Mark) nitriding process. This process differs from the generally employed double dissolution of ammonia process by using single dissolution of ammonia which results in surface hardnesses in excess of 60 HRc (HARDNESS ROCKWELL"C"to AS 1815-1992). The nitrided layer may be to a depth of about 0.6mm. The security bar has a typical balance of mechanical properties of 1100 (MPa) U. T. S at a minimum elongation of 8%.

Thus, the security bar is resistive to bending and there is no propensity for brittle failure.

A grille or assembly of the security bars of the invention comprises a horizontal and vertical arrangement of the bars welded into a mesh of required

aperture size. The mesh is contained within a frame to which the horizontal and vertical bar ends are welded with the ends protruding beyond the frame by about 60mm. The protruding ends are concreted into the surrounds of the opening to which the grille is applied.

The welding is preferably a Metal-Inert-Gas (MIG) welding process using stainless steel filler wire to AISI 316L.

In order to demonstrate the effectiveness of the steel bars of the invention, test pieces were subjected to machine hacksaw testing at various downloads and speeds. It was found that the bar could not be excised without frequent machine blade changes since the blades were dulled by losing both set and tooth depth immediately after commencements of attempts to cut the bars.

The weldments could not be cut. Owing to a 45% penetration of the weld puddle into the substrate, excisions through the remainder of the bar substrate were not successful-that is, the bars of a grill system could not be separate through the weldments since the parting tool contacted the fused substrate which does not permit separation by sawing.

The resistance to corrosion of the security bars of the invention is basically governed by the type of protection such as Electroless Nickel Plating, Zinc-Aluming, Galvanizing or Primer/Paint applications. An anticipated service life of 15 to 20 years or more can be envisaged by employing any of the abovementioned permanent protection methods.

Empirical tests as well as metallurgical/engineering theory demonstrate that the security bars of the invention are at least equivalent to the COMSTEEEL P8 security bar system.

A security bar of the hardened steel, having a diameter of 20mm and a 0.6mm nitrided layer having a surface hardness of about 60 HRc, cannot be

separated other than by'angel wire', a diamond and CBN (Cubic Boron Nitride) laced wire used in the Engineering Industry. The hard particles are attached to the parent wire by means of a polymer and tend to dull when used to cut nitride layers. To cut through such a 20mm bar would consume approximately 5 wires. In addition, the harmonics created within the parent wire emit a loud, screeching sound, which can be heard by supervisory personnel.

The nitriding layer depth does not depend upon the mass heat treated, but on the nitriding parameters, which are, being a surface induced process, the same for varying diameters.

An improved security grille could be constructed by creating a mechanical interlocking system of weft and weave pressed into the bars and connected by monel pins with the shaping of the substrate bars taking place prior to heat treatment.

Various modifications may be made in details of design and construction without departing from the scope and ambit of the invention for example, the nitridable steel material may be in the form of a plate or sheet which is treated in the same manner as the bar to provide a security sheet or plate. The security plate could be formed from a Bisalloy steel. The security material can be in any other convenient forms apart from bars and plate.