Strength, hardness and other steel plate mechanical properties vary widely depending on grade and thickness. Certain applications – such as construction equipment attachments and building infrastructure – require mechanical property levels to be greater than what can be achieved through carbon reduction or the addition of alloys.

Heat treating steel, which involves intense heating followed by cooling, alters the physical and sometimes chemical properties of steel, which thereby impact the material’s mechanical properties.

This article will look in-depth at four common heat treatment processes:

  1. Normalizing
  2. Annealing
  3. Quenching and tempering
  4. Precipitation-hardening

Steel plate suppliers – such as Leeco® Steel – typically stock a variety of grades that receive these treatments.

Mechanical Properties Enhanced Through Heat Treatment

Before exploring each of these processes, it is important to understand the differences between the steel plate properties that are targeted during heat treatments.

Strength: The load a material can bear, measured by yield point and tensile strength. Yield point is the maximum point of stress at which a material will permanently change shape, and tensile strength is the point at which a bent material will break.

Hardness: The level of wear resistance a material has, typically measured by a Charpy impact test.

Toughness: Not to be confused with strength or hardness, toughness refers to the ability of a material to absorb impact without breaking.

Ductility: How much a material can be stretched before becoming weak or brittle, measured by elongation – the percent the length of a material increases before breaking.

Machinability: The ease at which a material can be welded, drilled, bolted or machined.

While each of the common heat treatments have the same underlying process of intense heating and cooling, they alter above mechanical properties differently.

1. Normalizing

Normalizing involves heating steel plate to an extremely high temperature within a furnace. The plate is held at that temperature for a sufficient amount of time – typically a minimum of one hour per 1” of plate thickness – and is taken out of the furnace to air-cool at room temperature.

During this process, the grain structure of plate is altered to increase hardness, ductility, machinability and strength. These properties make normalized plate ideal for structural applications – such as for the construction of buildings, bridges or offshore structures – and for the construction of pressure vessels, which requires steel plate that can withstand high levels of stress while also being easy to form.

Some of the plate grades that Leeco typically supplies in the normalized condition include ASTM A516, A537 and A633, as well as API 2H 50. Each of these grades have relatively high yield points and tensile strength as a result of being normalized, detailed in the chart below.

Grade

Yield Point

Tensile

A516

30-38 ksi

55-90 ksi

A537

40-60 ksi

65-80 ksi

A633

42-60 ksi

63-100 ksi

API 2H 50

47-50 ksi

70-90 ksi

2. Annealing

Annealing, like normalizing, also involves heating steel plate to an extremely high temperature within a furnace. Where annealing differs from normalizing, however, is in the cooling process. Annealing has a more controlled cooling than normalizing, as plate is cooled within a furnace at a lower, below-critical temperature.

This process of controlled heating and cooling alters a plate’s grain structure to increase ductility and reduce hardness. With its enhanced strength and ductility, annealed plate is typically used in structural projects where the ability to bear heavy loads and stretch without becoming brittle is important, such as bridges, for example.

ASTM A829 4140 – a grade that can be supplied in the annealed condition – offers greater wear resistance than structural grades with lower carbon levels due to its higher tensile strength, as shown in the chart below.

Grade

Tensile

A829 4140

70-140 ksi

3. Quenching & Tempering

While normalizing and annealing have just one period of heating and cooling, quenching and tempering is a two-part process. The first part of the process, known as quenching, brings the material to a high temperature, usually between 1,500- and 1,650-degrees Fahrenheit. The plate is then rapidly cooled with water. After the material is quenched, it is re-heated again to a below-critical temperature, usually around 300 to 700 degrees, and air-cooled. This part of the process is known as tempering.

Quenching and tempering alters the material’s grain structure, resulting in increased hardness, toughness and strength. This hardness and toughness make quenched and tempered plate ideal for projects where durability is crucial, which typically includes construction equipment – dump trucks, trailers, cranes – pressure vessel tanks and mining equipment.

Some of the most common quenched and tempered grades stocked and sold by Leeco include ASTM A514, AR400 and AR500. While their heat treatment process is not regulated, AR400 and AR500 are typically quenched and tempered.

Grades like A514 have greater tensile and yield strength due to their increased toughness and strength than non-heat treated alloys grades, such as ASTM A572 Grade 50, which can be seen in the chart below.

Grade

Yield Point

Tensile

A514 ≥ 3/4” thick

100 ksi

110-130 ksi

A514 ≤ ¾” to 2.5” thick

100 ksi

110-130 ksi

A514 ≤ 2.5” to 6” thick

90 ksi

100-130 ksi

A572 Grade 50

50 ksi

65 ksi

4. Precipitation-Hardening

Like quenching and tempering, precipitation-hardening also involves two periods of re-heating and cooling steel plate. First, the plate is heated to a high temperature – between 1,000 and 1,300 degrees – then rapidly cooled. Once the plate cools, it goes through a process known as age hardening, where plate is reheated again at a lower, below-critical temperature and rapidly cooled.

Precipitation hardening increases the strength of plate and can increase toughness. The high strength and machinability of precipitation-hardened plate makes it commonly used in applications requiring the ability to withstand high stress, such as jack-up rigs, construction equipment, rail cars, truck frames, piping components and shipbuilding.

Grades that are precipitation hardened, such as ASTM A710 – which is stocked by Leeco – have high tensile and yield points as a result of their altered grain structure, shown in the chart below. These properties help A710 withstand the effects that harsh environments can have on steel.

Grade

Yield Point

Tensile

A710 Grade A

50-85 ksi

60-90 ksi

A710 Grade B

70 ksi

80 ksi

Choosing a Supplier for Heat-Treated Steel Plate

After determining whether a project requires heat-treated steel plate and, if so, what grade and treatment are required, purchasing teams must choose a supplier to fulfill their plate requirement. When looking for a steel plate supplier, here are some important factors that purchasing teams should consider:

  • Depth and breadth of plate inventory to ensure the supplier has the grade you need.
  • Proximity of distribution centers to order destination to reduce transit times and freight costs.
  • Quality management system, such as ISO certified systems, to ensure products are inspected to meet regulatory standards.

With 11 fully-stocked warehouses strategically located across North America, Leeco Steel can get you the plate you need, when you need it. Leeco is also ISO 9001 certified and performs quality checks throughout each stage of the ordering process to ensure customers receive high quality plate products.

Contact us or request a quote today to discuss your heat-treated steel plate needs.