GENERAL DESCRIPTION

VRN 400 abrasion resistant steel plates are heat treated to give an optimal combination of hardness, toughness and weldability. Depending on thickness and application, these properties are achieved through either thermomechanical rolling, quenching and tempering or quenching only. The steel is made to a fully killed fine grain practice.  It is calcium treated and desulphurised to achieve low sulphur levels and a very low inclusion content with controlled shape. These processes result in the steel having adequate toughness in addition to high hardness.

During heat treatment the entire plate is subject to a rapid, high volume water quench.  The high cooling rate ensures maximum exploitation of the alloying elements to give the required properties throughout the thickness of the plate.  In this way the composition can be kept sufficiently lean to provide a readily weldable product.  All plates are ultrasonically tested prior to heat treatment.
   

HARDNESS
Hardness of heat treated steel relates to the condition at the surface, whereas hardenability of the steel grade is a measure of the depth ot which the hardness is maintained in the steel.  To obtain the required hardness in thick plate, it is essential to increase the ability of the steel to be hardened.  Hardenability of a steel grade is determined by the chemical composition.  As the thickness of steel plates is increased, the alloy content has to be increased to obtain the required hardenability.

MECHANICAL PROPERTIES
The numbers in the grade designations generally indicate the nominal Brinell hardness values of the steel at the surface.  These steels are produced to meet specific nominal surface hardness requirements and not to conform to any tensile requirements.

There is however a correlation of hardness and tensile strength as set out below:

Brinell hardness :- 400
Approximate tensile strength (MPa) :-1 350

BENDING
VRN400 may be readily formed, provided the following precautions are met:

• for bending transverse to the rolling direction, a radius, R, of at least 3 times the plate thickness, t, should be  maintained;

• for bending parallel to the rolling direction, R should be at least 5 t;

• a sharp blade should never be used for bending operations;

• a die opening,W, of at least 8.5t should be used to ensure successful transverse bending, and at least
      10t for longitudinal bends;

• a U-bottom is recommended rather than a V-bottom for any bending where the included angle is smaller than 90^o.
     

 
MACHINING
Machining can be performed using high-speed tool steels and reducing cutting speeds to 50% of speeds used for normal carbon steels.

WELDING
VRN 400 is readily weldable.  Low hydrogen procedures must be used to avoid cracking in the heat affected zone.  In addition, low tensile strength consumables should be employed to minimise residual stresses.  Recommended pre-heating temperatures are shown in the table overleaf.  High heat inputs will reduce the hardness in the heat affected zone and must be avoided, especially in multi pass welding.

PREHEATING TEMPERATURES
The objective of pre-heating is to ensure a good microstructure in the heat-affected zone (HAZ) by reducing the cooling rate.

An excessive cooling rate is caused by:
-    insufficient heat input during welding
-    too low a temperature of the parent plate
-    too thick a plate

In such cases a brittle martensitic microstructure, which is susceptible to hydrogen cracking, could arise in the HAZ.  Increasing the temperature of the parent plate by pre-heating is usually the easiest way to overcome the problem.

The recommended values are given below.  They are based on minimum heat-input rates, and preheating temperatures should be increased at lower rates of heat-input (see minimum heat-input values).    

¹⁾ Only applicable if low Hydrogen levels in welding consumables can be guaranteed.  If hydrogen levels are 10-15ml/100g of weld metal, a minimum of 100^oC preheat is necessary.

Minimum heat-input values¹⁾  

¹⁾ Heat-input in welding is defined as:

                                Welding current (amps) x Welding voltage (volts)
   HI (kJ/mm)    =    --------------------------------------------------------------------------
                                  Welding speed (mm/sec) x 1 000
 

Hydrogen-level control
Low hydrogen welding processes are widely used.  These processes entail the selection of low-hydrogen consumables, and ensuring that manual metal-arc electrodes and sub-merged-arc fluxes are thoroughly dried in accordance with the manufacturers' specifications.  The plates must be free of moisture, oil or grease before welding commences.

Reduction of hardness in the Heat-Affected Zone (HAZ)
The desired properties of VRN 400 plates are achieved by heat treatment. High temperatures, such as during welding, will inevitably reduce the hardness of the plate in the HAZ.  Refer to the test certificate in order to determine the heat treatment procedure for a specific plate.

Limiting Heat-input
It is not possible to avoid a certain amount of hardness reduction in the HAZ during welding, but this can be minimized by limiting the heat-input to a predetermined maximum value.  If, for instance, a drop in hardness to a value of about 270 Brinell hardness (28 Rockwell C) can be tolerated, the maximum heat-input values, indicated in the following table, could be used.  These values can be exceeded if lower hardness values in the HAZ are acceptable e.g. in hidden corners.
   

• To be established by procedure test

Weld procedure tests
Before any critical welds are made, it is advisable to conduct a weld procedure test to check the hardness profile across the weldment.

RESIDUAL STRESS LEVEL
The residual stress in weldment is determined mainly by:

-    external restraint
-    fit-up
-    yield strength of the weld metal

External restraint
Since the method of installing wear-resistant plates usually induces a high level of external restraint, particular attention must be given to fit-up and yield strength of the weld metal.

Fit-up
Fit-up is important, especially in small or single-run welds, where the aim should be to keep the root gap below 0.4 mm.

Yield strength of the Weld Metal
The selection of the correct filler metal is essential to restrict the residual stresses in the weldments.  During cooling, either the parent plate or the weld metal has to yield to accommodate the shrinkage stresses.  Since the parent plate has a very high yield strength, it is imperative to use a filler metal with a low yield strength to prevent the generation of excessive residual stresses.

If it is considered essential to provide for abrasion resistance of the weld bead itself, it is advisable to initially deposit "soft" beads and to apply wear-resistant beads on the surface.

SHEARING AND PUNCHING
VRN 400 plates can be sheared in thicknesses up to 25 mm.  Because of the high hardness of VRN 400 compared to structural steel, the machine capacity should be decreased to 40% of the normal capacity.  Punching of VRN 400 is not recommended.

GAS CUTTING
Owing to the good weldability of VRN 400, the steel can be readily cut by oxy-acetylene flame.  Some distortion could be experienced as cuts are made because the low tempering temperature of the steel does not relieve all the internal stresses induced during the quenching process.  If subsequent bending is to be done, grinding of the flamecut edges is recommended.  VRN 400 can be successfully cut by plasma, laser and hydro jet cutting techniques.

HOT-WORKING
VRN 400 is normally tempered at temperatures below 425^oC.  To ensure that full hardness is retained, the steel should not be hot-worked at temperatures above 330^oC.  Refer to the test and analysis certificate for actual tempering temperature.  Hot working should be done at temperatures at least 30^oC lower than the actual tempering temperature.  The tempering temperatures are indicated on the test certificate.  Hot working of VRN 400 is not recommended as it is normally supplied in the as quenched condition.

EDGE CONDITION
Plates thicker than 12.0 mm normally have flamecut edges, whereas plates 12.0 mm and thinner normally have sheared or plasma cut edges.