The high strength quenched and tempered special structural steels N-A-XTRA® and XABO® from Thyssenkrupp Steel Europe are most commonly used in highly stressed welded structures requiring low weight and maximum strength.
The N-A-XTRA® and XABO® low-alloy quenched and tempered structural steels have a fine martensitic-bainitic microstructure. High strength is combined with excellent toughness and high fatigue resistance. N-A-XTRA® and XABO® steels have excellent surface quality and restricted tolerances. Hot strip mill production offers the ultimate advantage of +/- 0,2 mm thickness tolerances.
Quenched and tempered steels allows a reduction of the plate thickness and thus lighter buildings and structures. Light weighting is particularly important in commercial vehicle and mobile crane construction. This leads to clear economic and environmental advantages too.
Despite their outstanding strength, these special structural steels have excellent toughness and cold forming features. Processing and welding is easy.
N-A-XTRA® 700 is the most common quenched and tempered structural steel. It combines a high yield strength of at least 700 MPa with outstanding toughness even at temperatures down to -60 °c. Depending on the type of stress involved, using N-A-XTRA® 700 instead of S355 allows a 50 % reduction in plate thickness.
XABO®460 is the right choice for cold temperatures.
- Dimensions
- Welding
- Machining
- Bending
- More details
The most common high strength steel thicknesses and plate sizes available from our stock:
| Grade | Thickness (mm) | Width x length (mm) |
|---|---|---|
| N-A-XTRA® (M)700 | 6–30 12–70 80 100–120 | 2500 x 6000 2500 x 8000 2500 x 6000 2000 x 6000 |
| XABO® 460 | 10–40 | 2500 x 6000 |
N-A-XTRA® and XABO® plates can be welded by means of all prevalent methods.
Attention should be paid to prevention of the cold cracking behavior of the steel and the mechanical properties of the weld joint. The optimal working temperature and the cleanliness of the surfaces are both crucial.
Welding conditions have a crucial effect on mechanical properties of welding joints. Thermal effects of welding are not allowed to weaken the steel properties in the way the construction would not meet the requirements set. Mechanical properties should be achieved both welding metal and heat affected zone (HAZ).
A steel’s susceptibility to cold cracking can be estimated on the basis of its chemical composition. Particularly suitable for this is the carbon equivalent (CET).
CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 +Ni/40
Low CET-values are typical for the quenched and tempered special structural steels:
| Thickness (mm) | < 20 | 20 – 40 |
|---|---|---|
| N-A-XTRA® 700 | 0,31 % | 0,33 % |
According to experience, the risk of cold cracking tending to occur in the weld metal is large if the weld metal has a CET comparable to or higher than that of the parent metal. The increased CET values of the filler metals are relevant when calculating preheat temperatures.
An effective mean to avoiding cold cracking is preheating. It delays the cooling of the weld region and thereby slows the hydrogen effusion. Depending on the welding technique, the quenched and tempered special structural steel plates up to 20 mm can be welded without preheating. Also due to the reduced plate thicknesses compared to general structural steels, the costs for preheating prior to welding are much lower.
To avoid cold cracking, the hydrogen content in the weld metal should be as low as possible. In the case of root passes and single-pass fillet welds, the weld metal undergoes an alloying through dilution with the parent metal. As a result, the yield and tensile strength of the weld metal are increased. For this reason lower alloyed consumables are used for root passes and single-pass fillet welds in the case of high strength steels.
The properties in the heat affected zone of the weld undergo a change as a result of the temperature time cycle during welding. The parameter of cooling time t8/5 indicates the time needed for the temperature to pass from 800 °c to 500 °c.
Excessively rapid cooling of welded beads leads to a high hardness and the risk of cold cracking increases in the welding area. Too slow cooling, on the other hand, results to the situation where the weld metal’s properties don’t correspond to those of the parent metal. In addition the toughness of the heat affected zone may become lower.
Normal range for cooling time8/5 of N-A-XTRA® and XABO® high strength steel depending on welding technique:
| Cooling time t8/5 | |
|---|---|
| MIG / MAG welding | 5–15 s |
| manual arch welding | 8–20 s |
| submerged arc welding | 10–25 s |
N-A-XTRA® and XABO® quenched and tempered special structural steels are suitable for machining by all prevalent methods including, turning, milling, planning and drilling. Their higher strength means that lower metal removal rates are to be expected in comparison with the milder general structural steels.
When drilling it is necessary to avoid or minimize vibrations by:
- clamping the work piece firmly as close to the point of drilling as possible
- positioning the work piece and the drilling head as close to the machine column as possible
- using short drills and a short machine spindle
Guide values for machining N-A-XTRA® steel:
| Cutting speed (m/min) | D = 6 mm Rpm / mm/R | D = 8 mm Rpm / mm/R | D = 10 mm Rpm / mm/R | D = 12 mm Rpm / mm/R |
|
|---|---|---|---|---|---|
| N-A-XTRA®700 | 3–5 | 210/0,05 | 160/0,08 | 130/0,10 | 100/0,15 |
The forming of N-A-XTRA® and XABO® plates generally takes place in the cold state. The predominant forming techniques are cold bending on three-roll bending machines and press-brake bending in a 90° V-shaped die on press brakes.
It is necessary to take into account that the higher the yield strength, the greater the force required and the greater the springback. Furthermore, the formability of the steels must be expected to decrease with increasing yield strength. Larger minimum bending radius and greater support roll spacings are consequently used for three-roll bending machines, as well as increased forming die widths for the press brakes. If heat treatment is necessary, stress relief annealing is generally sufficient.
Minimum bending radius, die widths and roll spacings for cold forming of N-A-XTRA® 700:
| Thickness (mm) | Bending radius or die width (mm) | Roll spacing (mm) | ||
|---|---|---|---|---|
| I | II | |||
| N-A-XTRA®700 | t ≤ 6 6 < t ≤ 16 | 2 t 2 t | 2,5 t 3 t | 11 t 12 t |
I = Bending line transverse to the rolling direction
II = Bending line parallel to the rolling direction
It is presupposed that the forming zone, in particular the plate edges, is notch-free and that plate edges strain-hardened by shearing or hardened by thermal cutting are removed. The minimum bending radius apply instances where the bend axis lies in longitudinal direction. If the forming takes place transverse to the rolling direction, it is possible to reduce the given minimum bending radius.
More information about high strength steels can be found from following pdf-files:
- N-A-XTRA® and XABO® high strength fine-grain structural steels for new freedom in building lightweight special vehicles
- N-A-XTRA® and XABO® high-strength steels – Processing recommendations
- Datasheet N-A-XTRA®
- Datasheet XABO®
