Shotblasting – Recognising Steel Production Processes

Shotblasting of steel plate, profiles, construction, and fabrications is a process to clean, descale, provide a specified surface profile and edge break as a surface preparation. The process detail is subject to the hardness and start condition of the steel and it is vital to know these factors prior to shotblasting and before application of a coating or paint to maximise the adherence potential and corrosion control.

This is a document made up of five parts  on  “How Do You Make Your Coatings Stick Better?”

Section 1 – Specified Consistency
Section 2 – Measurement Terms
Section 3 – How To Measure
Section 4 – Cleanliness and Surface Profile

Section 5 – Recognising Steel Processes (for hardness of steel)

The effect of heat treatment is best explained by the different production processes or rolling process specifications that can be used in steel manufacturing the main ones being:

  • As-rolled steelRolling Mill
  • Normalised steel
  • Normalised-rolled steel
  • Thermomechanical rolled (TMR) steel
  • Quenched and tempered (Q and T) steel

Steel cools as it is rolled and the typical rolling finish temperature is 750°C, after which the steel cools naturally.  Steel produced through this process method is termed ‘As-rolled’. Structural sections generally achieve the required mechanical properties through this efficient production route, but plates usually require further heat treatment.

Normalising is the process where an as-rolled plate is heated back up to approximately 900°C, and held at that temperature for a specific time before being allowed to cool naturally. This process refines the grain size and improves the mechanical properties, specifically the toughness. It renders the properties more uniform, and removes residual rolling strains.

Normalised-rolled is a process where the rolling finish temperature is above 900°C and the steel is allowed to cool naturally. This has a similar effect on the properties as Normalising, but it eliminates a process. Normalised and Normalised rolled steels are denoted ‘N’

Thermomechanical rolled steel utilises a leaner chemistry, which requires a lower rolling finish temperature of 700°C to put the strength in, before the steel cools naturally. Note that greater force is required to roll the steel at these lower temperatures and that the properties are retained unless reheated above 650°C. Thermomechanical rolled steel is denoted ‘M’

Quenched and Tempered steel starts with an as-rolled plate, heats it back to 900°C and holds it at that temperature, as for normalizing, but then the steel is rapidly cooled or ‘quenched’ to produce steel with high strength and hardness but low toughness. The toughness is restored by reheating it to 600°C maintaining the temperature for a specific time and then allowing it to cool naturally (‘tempering’). Quenched and tempered steels are denoted ‘Q’.

After understanding the quality of steel, the steel process method, its hardness, combined with the surface profile specification, the blast process can now be created.

 

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