Several ASTM specifications are followed by an “M” that denotes it being an equivalent metric standard. While there is actually an A193M Grade B7 specification which is applicable to metric fasteners, this is not the case with ASTM A193 Grade B7M. This is a different grade of material than A193 Grade B7. So how exactly does A193 Grade B7M differ from A193 Grade B7?
There are some significant differences in both the mechanical properties and the testing requirements of A193B7M and A193B7. For instance, A193B7M has a minimum tensile strength of 100 ksi for diameters under 4”. The minimum tensile strength for A193B7 is 125 ksi for diameters under 2-1/2”, and 115 ksi for 2-1/2” – 4” diameter. Hardness is probably the most significant difference between A193B7M & A193B7. The maximum hardness of ASTM A193B7M is 235 HB or 99 HRB, while the maximum hardness of A193B7 is 321 HB or 35 HRC. The most notable difference from a manufacturing perspective is that when providing A193B7M bolts, every individual bolt or stud in the lot must be hardness tested. A193B7M fasteners will also require a different grade of heavy hex nut than A193B7, which would be ASTM A194 Grade 2HM.
While A193B7 is specified relatively often in the construction industry, A193B7M is a specification that is not seen as regularly. This makes it important to understand the differences between the two specifications so that the correct material is ultimately provided.
Estimado señor,
Superviso a una contratista y ellos argumentan que los esparragos con especificación A193 B7M pueden ser utilizados con las tuercas ASTM A194 Grado 2H, y que los esparragos con especificación A193 B7 pueden ser utilizados con las tuercas ASTM A194 Grado 2HM, que consecuencias puede tener si se utilizan asi?
Gracias por su atención.
@Jorge- A193 B7 se usa normalmente con tuercas A194 2H, y A193 B7M se usa normalmente con tuercas A194 2HM. Los grados “M” están sujetos a un mayor nivel de escrutinio y siempre deben usarse juntos a menos que el ingeniero del proyecto apruebe las sustituciones.
Mike,
Are the ASTM A193 B7M bolts (nut/screw) magnetic material? From what I can find in ASTM A193, B7 is listed under ferritic steels, which to my knowledge is magnetic. However, I find some contradicting answers when searching around, can it be both depending on the heat treatment and consequent microstructure? Your advise would be much appreciated.
@Solveig- Yes, A193 B7 and B7M material is magnetic. It is made from 4140 alloy steel which is a ferritic steel. There are demagnetization techniques, but normal, off the shelf B7 and B7M bolts will be magnetic.
How can we convert B7 to B7M ? what is the procedure of heat treatment?
@Usama- In order to change B7 to B7M, you would need to temper the bolts to a lower hardness. However, the stamping requirement for B7M is different than B7, so the parts would need to be re-stamped and 100% hardness tested. Your heat treater can talk you through the specifics of the heat treat/temper process.
Mr. Dane McKinnon,
Good Evening sir,
I am working in petroleum refinery. We use B7M stud/nuts for our Heat exchanger internal bolts (always submerged in hot process fluid). for external studs.nuts, we use B7 only. As we know B7M is having less strength but low hardness than B7, how it is helping us.
I understand we could not tighten the internals bolts (B7M in our case) and it should withstand in extreme process condtion i.e. should not fail leading to internal leak.
Dear sir,
how I can know this metal applied heat treatment after hardness testing
@Saddam- Your supplier should have material test reports and documentation to show how the material was manufactured. Regardless, the hardness testing should be completed after the final heat treatment.