Heat-treatable Plate Steel

Plates and pipes used for applications requiring superior strength and impact resistance are typically quenched & tempered. Examples include:

  • Crane booms and support structures
  • Mechanical equipment and machinery
  • Wear and armor plate
  • Pressure vessels and storage tanks

Applications for heat-treatable plate steel, from left to right: penstocks of a large hydroelectric facility, hydraulic mining shield, off- highway trucks used to carry highly abrasive rock

The goal for using higher strength steel for such applications is often weight reduction that comes with reduced plate gage. Molybdenum is an important alloying element in plate steel grades when the required yield strength exceeds about 500 MPa (The need for molybdenum alloying also depends on the plate gage and the steelmaker’s production facilities). Reduced structure weight brings many advantages, including lower material consumption (Figure 1), less welding (Figure 2), and reduced transport and hoisting costs. These advantages more than compensate for the increased material cost of molybdenum-alloyed steel.

Figure 1. Potential for thickness (and weight) reduction with increased yield strength.

Figure 2. Schematic of cost savings associated with Mo-alloyed HSS plate, exemplified by substituting S890QL grade for S355 grade.

Structural steel

High-strength, water-quenched and tempered structural steels can fulfill severe strength and toughness requirements, e.g. those required for mobile cranes. These steel grades have minimum yield strengths ranging from about 550 MPa to about 1100 MPa. In fact, steels with minimum yield strengths of 1100 MPa represent the apex of high-strength structural steel development. Water-quenched and tempered structural steels with minimum yield strength of 500 MPa and a low carbon equivalent are also available and are being used in offshore engineering, for example. Producers adjust the mechanical property combinations of these steels by controlled tempering of the quenched plates at temperatures up to 700 °C. These steels have excellent combinations of high strength and good toughness. Their use in heavily loaded and sometimes safety-relevant structures demands brittle fracture resistance at subzero operating temperatures.


The chemical compositions of these steel grades must be designed to obtain the mechanical properties necessary for each specific application. High-strength quenched and tempered structural steels usually contain less than 0.2% carbon and a maximum of 2% manganese. They may also contain alloy additions of elements that retard the diffusion-controlled transformation process and thus increase their hardenability. Molybdenum, chromium and nickel are examples of such additions.

Standard high-temperature structural steels
GradeAlloy conceptMin. yield strength
(MPa)
Tensile strength
(MPa)
Min. A5 (%)Min. CVN
of 27 J at:
S550QL
S620QL
S690QL
CrMoB 550
620
700
640 – 820
700 – 890
770 – 940
16
15
14
-60°C
S890QL
S960QL
S1100QL
CrMoNiV 890
960
1100
940 – 1100
980 – 1150
1200 – 1500
11
10
8
-40°C

The plate is reheated to temperatures above Ac3 temperature during heat treatment. It is important that the plate temperature is uniform from its surface to its core, so that the resulting mechanical properties are uniform. The plates are quenched with pressurised water sprays to maximize the cooling rate and guarantee transformation of the microstructure into martensite or bainite through the thickness. The tempering process that follows quenching is a critical factor controlling the plate’s final mechanical properties. A high density of dislocations and high internal stresses characterize the martensitic microstructure in the as-quenched state. This results in a very hard and strong material, but it is one with little toughness. Tempering reduces these internal stresses and the dislocation density, somewhat reducing strength but greatly improving ductility and toughness. Too much tempering reduces the strength to insufficient levels. Molybdenum efficiently mitigates this effect through its contribution to solid-solution strengthening and in concert with other elements such as chromium and niobium that cause secondary precipitation of complex carbides. By forming these carbides, molybdenum efficiently delays the loss of strength during tempering and improves fracture toughness.

Weldability

Carbon strongly affects a steel’s weldability. Increasing carbon content increases a steel’s strength but reduces its ductility and toughness. Other alloying elements may have similar effects, but with different strength. An empirical factor called the “carbon equivalent” is used to assess an alloy’s susceptibility to welding problems. The carbon equivalent (CE) is probably the most important criterion to judge a steel grade’s weldability. Many definitions of this parameter exist, but two are commonly used for quenched and tempered steel plate, CEV and the CET, defined as:


CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Cu+Ni)/15
CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40


The factors show that it is particularly effective to reduce carbon content of a steel to improve its weldability. This is simultaneously improves toughness but decreases strength. Therefore strength must be regained by using other alloying elements and thermomechanical (TM) treatment during plate production. To comply with a maximum specified carbon equivalent, the alloy composition must be optimized according to the weighting factors in the CE criterion. Thermomechanical treatment combined with accelerated cooling can produce steel with a yield strength as high as 690 MPa. For a specified strength, TM steel always has a lower carbon equivalent than QT steel. However, for a given strength the maximum thickness for TM steels less than that for QT steels, especially at higher strengths. See the table below for comparisons.

Comparing alloying concepts for high-strength TM and QT plate
GradeMax. gageAlloy content (mass %)
CSiMnCrMoNiCuNbVCEV
500 TM 35 mm 0.11 0.45 1.65 - - - - 0.05 0.07 0.41
500 QT 70 mm 0.10 0.30 1.40 0.15 0.20 0.60 0.20 0.025 0.05 0.47
690 TM 25 mm 0.08 0.30 1.80 - 0.30 0.50 0.30 0.03 0.05 0.51
690 QT 70 mm 0.13 0.30 0.90 0.40 0.40 1.00 0.25 0.025 0.04 0.53

Figure 3 shows the strong effect of the time to cool from 800 to 500°C after welding (ΔT8/5) on the properties in the weld's heat-affected zone (HAZ). While the carbon content controls the plateau hardness of the martensite at very short cooling time, the CE determines when martensite forms during cooling. With increasing CE, the critical ΔT8/5 shifts to longer times and falls within the working range typical of SMAW and MAG welding processes used for on-site welding of plate sections. Grades with a comparably high CE demand special precautions for welding, including pre-heating of the weld zone. The process window becomes narrower as the CE increases (Figure 4). Welding with too much heat input reduces both strength and toughness, whereas too little heat results in excessive hardness and increased risk of cold cracking in the HAZ.

Figure 3. HAZ hardening of various high-strength plate grades as a function of cooling time ΔT8/5 after welding.

Welding consumables that produce tensile properties overmatching those of 690 or 890 MPa QT-steel require relatively high alloy content. A combination of 1-2.5% Ni, 0.5-1.5% Cr and about 0.5% Mo is typical. The resulting weld's chemical composition and as-cast microstructure often render it more susceptible to hydrogen-induced cold cracking than the HAZ of the ultra-high strength parent material. To avoid cracking, preheat and interpass temperatures must be adapted to the weld metal. Lowering the carbon equivalent does not allow dropping these welding precautions. Consequently, high-strength parent material with reduced carbon equivalent, as achieved by TM rolling, need essentially the same precautions as the more highly alloyed QT steel.

Figure 4. Welding process windows for three high-strength plate grades.

Chemical composition of wear-resistant special structural steels
Target hardness (HB)Max. plate gage (mm)Chemical composition (max. %)Typ. CET (%)
at gage
CSiMnCrNiMo8 mm40 mm
400 100 0.20 0.80 1.50 1.00   0.50 0.26 0.37
450 100 0.22 0.80 1.50 1.30   0.50 0.38 0.38
500 100 0.28 0.80 1.50 1.00 1.50 0.50 0.41 0.41
600 40 0.40 0.80 1.50 1.50 1.50 0.50 0.55 0.55

The characteristic wear type in most applications is ploughing that leads to abrasive wear. In this type of wear, a hard, abrasive material like sand scratches the plate surface. High hardness is one important feature for good wear resistance. Higher toughness also improves wear resistance and reduces material loss.

 

Pressure- vessel steel

Low-alloy chromium-molybdenum steels (1 to 3 % Cr and 0.5 to 1 % Mo) are typically selected for the fabrication of heat exchangers and process reactors.

Standards for pressure-vessel steels
Steel gradeASTM/ASME DesignationEN 10028-2 Designation
1Cr ½Mo A/SA387-12-Cl. 1/2 13CrMo4-5
1¾Cr ½Mo A/SA387-11-Cl. 1/2 13CrMoSi5-5
2¼Cr 1Mo A/SA387-22-Cl. 1/2
A/SA542-A/B-3/4/4a
10CrMo9-10
12CrMo9-10
2¼Cr 1Mo ¼V A/SA832-22V
A/SA542-D-4/4a
13CrMoV9-10
3Cr 1Mo ¼V A/SA832-23V
A/SA542-E-4/4a
12CrMoV12-10

Vessels requiring hydrogen resistance at elevated temperatures and high pressures (e.g. hydrotreaters, hydrodesulfurizers and hydrocrackers), require plate with sophisticated specifications. Petrochemical process reactors must operate safely under these conditions for the longest possible campaign times. Yield strength at room and elevated temperatures, creep reisistance, and impact energy are all important mechanical properties for these applications. Chemical composition and plate thickness are not the only factors that influence materials properties. Heat-treatment conditions during plate production (normalizing, tempering, quenching, tempering, and stress-relief annealing) all affect properties. Vessel manufacturing processes (hot forming, welding) also can play a decisive role in determining the final properties of the material. In addition, the material properties can change due to thermal effects during long-term service of the pressure vessels.