AU709722B3 - Security bars - Google Patents

Description

P00012 Regulation 3.2 Revised 2/98

AUSTRALIA

Patents Act, 1990

ORIGINAL

COMPLETE SPECIFICATION PETTY PATENT TO BE COMPLETED BY THE APPLICANT NAME OF APPLICANT: AG SECURITY PTY LIMITED.

ACN 081 379 160 ACTUAL INVENTOR: ADDRESS FOR SERVICE: Peter Maxwell Associates Level 6 Pitt Street SYDNEY NSW 2000 INVENTION TITLE: DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO: SECURITY BARS PP 8361 28 January 1999 Australia The following statement is a full description of this invention including the best method of performing it known to me:- This invention relates to security bars for use, for example, as bars or grilles in windows, doors and other openings to prevent illegal egress or ingress in custodial institutions.

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 as COMSTEEL P8. Such security bars may have various cross-sections such as circular, rectangular, square or polygonal.

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 documented in the past.

29/3/99 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 assembled by welding and which will respond to impactive forces like a spring and not deform or fail in a brittle mode.

According to one aspect of the invention there is provided a security bar formed from a nitridable steel which is subjected to austenitization followed by oil quenching and then tempered to provided a heat treated bar which is then nitrided to provide a security bar having a hardness in excess of 75 HR c According to another aspect of the invention there is provided a method of preparing a security bar comprising the steps of:selecting a nitridable steel bar; (ii) subjecting the nitridable steel bar to austenitization, quenching and tempering to provide a heat treated bar, and (iii) nitriding the heat treated bar to provide a security bar having a hardness in excess of 75 HR c 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 have in common: a Carbon content between .30 and (ii) a varying Aluminium content of 0.2 to 2.0% (The latter being typical of BS En 41 B) (iii) a Manganese content of 1.2% max.

(iv) Chromium from 0.7 to 1.1% a Molybdenum content of up to 0.45%, and (vi) a Silicon content of up to 0.35% Nickel strengthens the substrate, however, does not form stable nitrides. The family of nitridable steel grades is 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 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 bar to about 880 0 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 0 C for about 1.5 hours in an air-recirculation furnace.

The nitriding step may be carried out by using the BRISTUFF (Trade Mark) nitriding process. This process differs from the generally employed double dissolution of ammonia by using single dissolution of ammonia which results in hardnesses in excess of 75 HRc (HARDNESS ROCKWELL to AS 1815-1992). The nitrided layer may be in the order of 2mm thick with a 20mm diameter bar. The security bar has a typical balance of mechanical properties of 1100 (MPa) U.T.S at a minimum elongation of 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 which are welded into a mesh of required aperture size. The mesh is contained in a frame to which the horizontal and vertical bar ends are welded with the ends protruding beyond the frame by about 60mm which 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. However, owing to a 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 is impervious to separation attempts 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 invention, having a 2 [mm] Nitriding layer of HRc on a bar diameter of 20 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 Polymere and tend to dull when used to cut nitride layers.

To cut through a 20 [mm] AG 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.

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

An improved security grill 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.

7 Various modifications may be made in details of design and construction without departing from the scope and ambit of the invention.

Claims (3)

1. A security bar formed from a nitridable steel which is subjected to austenitization followed by oil quenching and then tempered to provided a heat treated bar which is then nitrided to provide a security bar having a hardness in excess of 75 HRc (HARDNESS ROCKWELL

2. A security bar according to claim 1 having a nitrided layer of about 2mm.

3. A security bar according to claim 1 wherein the nitridable steel includes:- 0.30 to 0.45% carbon, (ii) 0.2 to 2.0% aluminium, (iii) 1.2% maximum manganese, (iv) 0.7 to 1.1% chromium, 0.45% maximum molybdenum, and (vi) 0.35% maximum silicon. Dated this 5 day of March 1999. AG SECURITY PTY LIMITED By their Patent Attorneys PETER MAXWELL ASSOCIATES