Steel Shot & Grit Hardness

As a follow-up to the SAE specifications to identify the different types of shot peening media types, there are also specifications that specify the hardness of the Steel Shot or Steel Grit. Hardness is reported in the Rockwell 'C' range and is stated as Rc or HRC with higher numbers indicating a harder grade. SAE specifies that "ninety percent of random hardness checks performed on a representative sample shall fall within the ranges" and that the standard deviation allowed is +/- 3 HRC.

The typical ranges are as follows:
40 to 51 HRC (KGP)
47 to 56 HRC (KGB)
54 to 61 HRC (KGL)
61+ HRC (KGH)

Generally, the softer grades will round over more quickly making it better for removal of oxides and cleaning. Harder grades will maintain the angular shape (with Steel Grit) and are best for stripping and etching applications.

Blasting with Steel Shot

Steel Shot and Steel Grit are some of the most effective media due to their extreme re-usability and lack of dust generation during blasting. These two types of media are used most in wheel blasting equipment. In this process, the media is literally thrown against the parts and then recycled again and again. Unlike air blast (i.e., sandblasting) systems, wheel blasting does not require an air compressor and can be used with any size of Steel Shot or Grit.

Air blasting with Steel Shot and Steel Grit requires a little more consideration. If a cabinet system is being used, the steel media will be re-used many times but the type of blasting (suction or pressure), hose size and durability and nozzle type and size will determine what grades or sizes of steel media that can be used.

If a portable air blasting system is used, the size of the fittings, hoses and nozzles are also important in identifying which grades can be used. Re-usability can be a challenge as there typically is no system to automatically collect and recycle the media. The most important consideration concerns the operator and the weight of the hose while blasting. Steel Shot and Steel Grit can weigh more than three times as much as other blast media. The weight of the blast hose filled with media can fatigue the operator very quickly. Finally, pushing heavy weights (a 50 foot hose filled with steel media) will require lots of air pressure and air volume.

Abrasive Blasting Media - Steel

This final installment of the Abrasive Blasting Media series will discuss the merits of using both Steel Shot and Steel Grit for blasting. Both of these media types are produced from scrap steel which is melted, modified and then solidified into a generally round shape (Shot) or angular particles (Grit).

Steel Shot produces a very smooth finish on the surface of metals. The rounded shape does not strip a surface very effectively but rather peens the surface to produce a clean, smooth and (sometimes) polished finish. The smaller the shot the better the polish but the less aggressive the peening operation. The peening action of the shot is also used to impart compressive strength or hardening to metals.

Steel Grit is used when aggressive stripping action is required. Steel is softer than Aluminum Oxide so will not fracture as easily but the higher density produces a forceful blasting process. Smaller sizes and softer formulations of grit are used when lighter cutting is required while larger sizes and harder steel formulations will maintain the particle angularity and a high strip rate.

Since both Steel Shot and Steel Grit have a high bulk density of about 250 pounds per cubic foot, special or modified equipment is necessary to blast these media using air pressure alone. Steel is often used in wheel blast operations where the media is propelled by a wheel using controlled force.

Steel Shot & Air Blasting

Air blasting with either Steel Shot or Steel Grit provides a couple of unique challenges. Because of the weight of the media (2-3 times 'standard' abrasive blast media), it is important to check with the manufacturer to make sure that your blasting system is designed to handle this media. Many pressure systems are OK but siphon or suction systems will probably have some issues.

Clean and dry (and I mean dry) air is critical for air blasting with steel shot or grit. If moisture gets into the pressure pot, the steel can rust over time reducing the effectiveness and requiring more frequent replacement of the media. If the steel media is left in the pot overnight with moisture you will probably need to purchase a new blasting pot.

All-Time High for Steel

In a February post I explained the origins of the steel scrap surcharge. Basically, the base price of steel shot and steel grit is fixed but a surcharge is added (and changed each month) to bring the price of steel to current market value.

Well, the demand for steel in both Asia and Eastern Europe continues to grow and much of the scrap is being sent overseas to satisfy this demand. Obviously, increasing demand leads to increased pricing and even product shortages.

The current scrap surcharge is up about 500% in the past 5 years and has nearly doubled in the past six months. In addition, the lead times are continuing to extend for certain sizes and grades. Unfortunately, it does not look like this is going to be a short term problem unless steel manufacturers can figure out a way to make a lot more steel.

We're at an all-time high for steel prices but I fear we will continue to say the same thing many times in the near future.