Shearing
Macsteel VRN T690 steel plate can be cold sheared up to and including 25mm thickness if the capacities of the shear and the shear knives are adequate.  A shear capable of cutting 38mm carbon structural steel is required to shear 25mm plate of MacsteelVRN T690 steel.  The capacity of smaller shears would be down rated in proportion, i.e., the capacity is decreased about 33 per cent when Macsteel VRN T690 is sheared.

The clearance on the shear knives should be less than the conventional 5 per cent of plate thickness, usually about 2 per cent.  A compromise clearance to accommodate all plates from 6mm to 25mm in thickness would be about 0.4mm.  Knives should be kept sharp to avoid ragged edges.

Punching
Holes may be punched in Macsteel VRN T690 plates up to and including 12mm in thickness as long as the hole diameter is greater than the plate thickness.  Punch and die clearance should be close, as described for shearing, and the tools should be kept sharp.  Greater thicknesses can be punched;  however, excessive punch wear may result.

Machining
Macsteel VRN T690 steel plates can be machined with conventional equipment using either high-speed steel or carbide tooling.  Because of the relatively high hardness of Macsteel VRN T690 steel as compared to carbon structural steel, the cutting speeds should be about 30 per cent less in order to obtain reasonable tool life.

A coolant should always be used if available as an aid to tool life.  When extensive machining is necessary on torch cut edges, it may be found essential to soften the edges.  This can be done by tempering the entire plate, or only the edges, in a temperature range of approximately 540^o to 595^oC.

Torch cutting
Macsteel VRN T690 steel plate can be cut with conventional oxygen fuel gas equipment without the necessity of preheating or postheating, employing the same practices used for soft carbon steels.  Stack cutting of plates, however, should be avoided because of excessive heat inputs required.

As with any structural steel, the smoothness of the cut is affected by scale on the surface of the plates.
Plasma-jet torch cutting is suggested wherever available and is advantageous because of the high cutting speed, the smoothness of the cut, and the shallow heat-affected zone produced.
On multiple cuts, balanced torch settings will help avoid distortion.  

WELDING

General
The Macsteel VRN T690 low alloy grades of steel were developed to be easily weldable.  High strength in structural steels may be achieved either by addition of alloying elements or by thermal treatment, as with the Macsteel VRN T690 steels.  They are quenched and tempered and contain a minimum of alloying elements which render them easily weldable.

Most of the well known fusion welding processes may be employed on Macsteel VRN T690 steel.  Fusion welding involves depositing molten weld metal in order to achieve a joint.  The chemical analysis and the cooling rate of the weld metal can be controlled. However, the region directly adjacent to the weld on either side, known as the heat-affected zone (HAZ), experiences a thermal cycle, ranging from unaffected parent plate to near melting at the fusion boundary. Since the chemical analysis of the parent material is unalterable, it is important to take care of the thermal cycle of the HAZ. When welding Macsteel VRN T690 plate a number of general factors have to be borne in mind.

The main source of concern in welding these steel is hydrogen induced cold cracking. By minimising the sources of hydrogen and by avoiding the formation of a crack-sensitive microstructure and also by keeping stresses below certain limits, hydrogen induced cracking can be avoided. In addition, for full strength buttwelds a suitable welding consumable must be selected.

Sources of hydrogen
Dirt, grease, paint, moisture, rust, etc., on the plates to be welded should be positively removed.
The welding consumables should be of approved quality and should be clean and dry, during both storage and usage.  Different welding processes have different inherent hydrogen potentials.  When welding with the processes of highest hydrogen potential, greater care is required than welding with processes of lower hydrogen potential.

Avoidance of crack sensitive microstructure.
The microstructure in the HAZ is determined by the steel composition as well as the local cooling rate.
A knowledge of the steel composition is therefore essential for proper selection of welding parameters.  The use of the Carbon Equivalent formula gives an indication of the degree of care required.

The CE of Macsteel VRN T690 ranges between 0.37 and 0.54.  In order to avoid a crack-sensitive microstructure, a suitable cooling rate in the HAZ must be maintained after welding.  Generally this involves using pre-heating and controlled values of heat input during welding.  The particular values of pre-heat and heat input depend upon a number of factors but average values are quoted below in Table 1.  Heat input can be calculated from the formula.

                       Welding volts (V) x welding current (Amps)                kJ
HI        =        -------------------------------------------------------------      =    --------
                                1000 x welding speed mm/sec                           mm

                              Mn           Cr + Mo + V             Ni + Cu
CE    =        C  +  -------  +  ----------------------  +  ---------------
                                6                      5                           5

It should be emphasized that the values of pre-heat and heat input quoted in Table 1 are average values.  Factors such as restraint, welding position, edge preparation, service conditions (type of load), etc., may influence the particular values elected.

Welding stresses
In general it may be stated that joints of high restraint need more care when welding than joints of low restraint.

This is of particular importance when using a strength filler material in order to achieve a full strength joint.
The molten weld metal contracts upon cooling if free to do so, causing distortion, or creating welding residual stresses of yield point magnitude if free contraction is hampered.  Values of high residual stress are conducive to distortion during subsequent machining operations and cracking in the weld or HAZ during and after welding.  Welded structures should be designed for welding in order to minimise the volume of weld metal which should be deposited in the shortest possible time to effect the greatest economy.

Summarising, it may be stated that Macsteel VRN T690 is a weldable quenched and tempered steel which may be successfully welded with the common fusion welding processes provided certain precautions are taken.  The level of hydrogen in the deposited weld metal should be kept to a minimum, the cooling rate must be slow enough to avoid the formation of crack-sensitive microstructures in the HAZ but not so slow that excessive grain growth occurs. Pre-heating and controlled values of heat-input are therefore required.

Welding consumables
The American Welding Society Classification AWS EXXXX is a very practical and useful system.  British and South African equivalents may be found in BS 639 and SABS 455 respectively.  For practical reasons, only the AWS classification groups are quoted here.

Matching strength consumables are suggested for welding Macsteel VRN T690 although lower strength consumables can be used. Consumables should conform to one of the following specifications:  S.M.A.W.: A.W.S. A5,5, E 9018 or E 11018; G.M.A.W.: A.W.S. A5,28, ER 90S or ER 100S
 
AVAILABLE CONSUMABLES FOR WELDING MacsteelVRN T690   

Heat input schedule

Effect of prolonged exposure during stress-relief on mechanical properties:
12mm plate Macsteel VRN T690
 

Heat input schedule for all grades 

* The combined thickness of a joint is the total thickness (mm) of the plates meeting at the joint line.