What's the Difference Between Hot Rolled Steel & Cold ...

Hot rolled steel or cold rolled steel – which to choose? One of the crucial parts of an engineer’s job is picking the right materials for their application. There’s already a lot of types of metal to choose from. Each with its own advantages and uses. Some types of steel make an excellent fit for home appliances, others for the automotive or marine industry, gas tanks, constructions, etc.

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Still, there’s one more distinction to make. A material grade with the same chemical composition may have varying qualities depending on the manufacturing method. Our customers have asked for the difference. So here it is.

Hot Rolled Steel

Hot working is more widely used compared to cold working because it needs less force and energy. It is used in compressive forming methods like rolling, metal extrusion, forging, etc.

Hot Rolling Method

Hot rolling takes place in temperatures above the material’s recrystallisation temperature. In case of steel, the temperatures rise above 1000°C.

The starting material is usually steel billets or slabs. First, they are heated above the aforementioned temperature. The next step is feeding them to the rolling machinery. Continuous rolling gives the desired final shape – a metal sheet (3 mm and upwards) or profile.

Hot Rolled Steel Properties

As it is easy to form metal in high temperatures without any extra delays, it is possible to produce it in larger quantities than cold rolled steel. This keeps the market price of hot rolled steel lower. The steel cools at room temperature. This is known as normalising. It changes the material’s microstructure in a way that results in increased ductility and toughness. Ductility is especially important when forming the material (e.g sheet metal bending) to give it the required shape for your needs.

Hot rolled steel, though, doesn’t have the best quality. It shrinks slightly during the cooling process. This leaves the metal with internal stresses. The results are non-uniform measurements and some distortions. The material’s dimensional tolerances can vary between 2…5%. Also, the surface has a scaly finish. This is a kind of oxide that forms at high temperatures, known as mill scale.

It is easy to identify hot rolled products by touching the surface because of the uneven finish but it is also lacking an oily film. With steel bars, the corners are rounded.

Hot Rolled Steel Uses

Hot rolled steel is a good choice when tight tolerances are not of utmost importance. There are many fields where that is the case. Its great advantage in price matters more than precision. Some common uses for hot rolled steel are:

• Constructions
• Pipes and tubes
• Truck frames
• Doors and shelving
• Railroad tracks
• Railroad car parts

Cold Rolled Steel

Cold working is a metal forming method that has many advantages over hot working. Technically, cold working includes cold rolling and cold drawing. The former is a process used with sheet metal. The latter finds use with rectangular and round bars.

Cold Rolling Method

As opposed to hot rolling, cold rolling occurs with the metal below its temperature of recrystallisation. This is still only half the truth. The whole process starts out like hot rolling to give the initial shape without much resistance. After that, the metal is left to cool at room temperature.

The half-products are then fed to cold reduction mills. The metal is rolled to the thickness of 0.5…3 mm in case of mild steel and 0.5…5 mm in case of stainless steel. The material is cooled by the use of oil which also acts as a lubricant during the rolling process. As the metal sheet gets thinner between the rolls, its speed increases. That would mean material wear and deformation if an oil film wasn’t there to minimise the contact. Therefore, cold rolled steel is identifiable by an oily and smooth surface.

As the working takes place in temperatures below the rescrystallisation temperature, strain hardening occurs. The rolls induce plastic deformation. Thus, the yield strength of cold rolled steel is higher than that of hot rolled steel. As an example, a hot rolled steel product may have a yield strength of 235 MPa. In comparison, a cold rolled steel product with the same chemical composition has a yield strength of 365 MPa.

The main advantages of cold working are:

• Accurate finished dimensions
• Clean surface
• Greater strength properties

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Cold Rolled Steel Uses

Although cold rolled steel is more expensive than hot rolled steel, the aforementioned advantages make it useful for many applications. The finished products need less additional surface finishing to achieve a good result, as the surfaces are already smooth. Examples of cold rolled/cold drawn steel uses include:

• Metal furniture
• Structural parts
• Home appliances
• Water heaters
• Metal containers
• Fan blades
• Frying pans
• Computer cabinets

When choosing the right material for your product, make sure to understand the difference of these two material manufacturing methods. There is no point in spending more money on something you don’t actually need. Therefore, hot rolled steel is the better choice when the demands aren’t high. Otherwise, go with cold rolled steel. It has the answers that hot rolled steel doesn’t.

When you are first looking to source a stainless steel pipe, whether it’s for a desalination plant, an oil rig or even a nuclear power station, one of the first questions you need to ask yourself is ‘do I need a seamless, welded or forged pipe’? The three types all have different advantages and are therefore suited to different applications and environments. An abundance of factors should be taken into consideration when choosing what is right for a particular project.

Engineers will more than likely instinctively know the answer to that question, but let’s spend some time exploring these seamless, welded and forged pipes and their various properties.

Let’s start with seamless pipe. As the name suggested, a seamless pipe is one without any seam or weld-joint.

Manufacturing and applications:

Seamless pipes can be manufactured using various different methods, which is very much dependent on the diameter required, or the ratio of the diameter to the thickness of the wall. In general, the manufacturing process of seamless pipe starts with raw steel being cast into a more workable format – a hot solid billet. This is then stretched out and pushed or pulled over a form. This hollow pipe then undergoes an extrusion process whereby it is forced through a die and mandrel. This works to increase the inside diameter and reduce the outside diameter.

Seamless steel pipe is regularly used in the transportation of fluids such as water, natural gas, waste and air. It is also regularly required in many high-pressure, high-corrosive environments such as in the oil & gas, power generation and pharmaceutical industries.

Advantages:

1. Strength: Seamless pipes come with the obvious advantage of having no seam and therefore no chance of a weak seam. This means that typically, seamless pipe can withstand 20% more working pressure than welded ones of the same material grade and size.
2. Resistance: The lack of seam means that seamless pipe can offer higher resistance to corrosion because there is less opportunity for things like impurities and defects that can more commonly occur along a weld.
3. Less Testing: It goes without saying that seamless pipes don’t require testing for weld integrity – no weld means no test!

Welded pipe comes in three forms: welded on the outside diameter, welded on the inside diameter or welded on both sides. The commonality is that they all have a seam!

Manufacturing and applications:

The manufacturing process of welded pipe starts by rolling out a steel coil to the desired thickness to form a flat strip or plate. This is then rolled, and the seam of the resultant tube is welded in a chemically neutral environment.

In regard to what type of steel is weldable, austenitic steels are generally the most weldable whereas ferritic steels are weldable in thin sections. Duplex steels are now regarded as fully weldable, but they do require more care than austenitic steels.

Welded pipe manufacturing techniques are thought to have greatly improved over the past few years. Arguably the most significant advancement being the development of using high-frequency electric currents as a welding technique. This has greatly improved the ability of welded pipe to avoid corrosion and seam failure.

While it is correct that the seam in a welded pipe makes it theoretically weaker, manufacturing methods and quality assurance procedures are far more superior these days. This means that as long as a welded pipe’s specified tolerances for temperature and pressure aren’t surpassed, there is no reason why is shouldn’t perform just as well as a seamless pipe in a wide array of industries.

Advantages:

1. Cost: A big advantage of welded pipe is that it is the least expensive of all pipe types and it is much more readily available.
2. Consistency: It is generally accepted that the wall thickness of welded pipes is much more consistent than that of seamless pipes. This is because the manufacturing process starts with a flat sheet of steel.
3. Surface quality: The avoidance of the extrusion process also means that the surface of a welded pipe can be much smoother than a seamless pipe as well.
4. Speed: Shorter procurement lead time is required for welded pipe due to the manufacturing process being simpler.

Forging steel is a metal forming process that shapes metal using compressive force and extreme heat and pressure.

Manufacturing and applications:

The manufacturing of a forged pipe starts with a piece of steel (whether that’s 6% Moly, Super Duplex, Duplex, Stainless Still, Nickel Alloy) being placed between an upper and lower die. Heat and pressure mould the steel into the desired shape and it is put through a machining process where it is finished to meet all of the required specifications.

This complex manufacturing process does result in higher cost implications for forged pipe.

The many advantages of forged pipe mean that it has a number of different applications in a diverse range of sectors such as oil, gas, hydraulic machinery, fertilisation and chemical. The fact that forged steel has no seam or welding point allows it to successfully contain potentially harmful or corrosive substances and their fumes. Therefore, it can be used in many heavy industries.

Advantages:

1. Strength: Forging pipe usually results in a strong and very reliable end-product because forging causes the grain flows of the steel to alter and align. In other words, the steel becomes more refined and the pipe’s structure is changed unequivocally resulting in sheer strength and high impact resistance.
2. Longevity: Forging excludes the potential for porosity, shrinkage, cavities and cold pour issues.
3. Economical: It is generally accepted that the process of forging is very economical as no material is wasted.
4. Flexibility: The steel forging process is very flexible and can produce many different pipes of varying sizes.

Whatever variety of seamless, welded or forged pipe you require, rest assured that Special Piping Materials is able to source and supply all sizes of seamless pipe, welded pipe and forged pipe. Our vast global network will be able to find and stock whichever type you need, in whatever material grade – be it super duplex, duplex, nickel alloy, 6% Moly or stainless steel. The trusted mills that we work with are highly efficient and specialists in their respective fields, ensuring that your products are exactly how you need them.

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