Difference between ASTM A194 Grades 2H and 2HM

What makes 2HM different from 2H? Does the M stands for ‘Metric’?
Although in many cases ‘M’ stands for Metric, in this case it is not so.

Nuts of Grade 2H indicate that the alloy steel used for nut is quenched and tempered whereas in case of Grade 2HM the nuts are additionally tested 100% for hardness. Both these nuts can be manufactured by means of any of the processes - hot forging, cold forging or machining but the material must be heat treated.

2H heavy hex nuts are used in high temperature and high pressure services, with adequate strength and engineering to withstand extreme conditions. 2HM nuts are also produced in a similar way, yet have low proof load values and hardness. So, while 2H nuts can be substituted for 2HM but vice versa may not be true.

The similarities and differences in these two grades are:
  • Chemical composition of both grades 2H and 2HM is same. Refer https://nuboteck.blogspot.com/2020/05/api-20e-bolting-astm-a194.html
  • The main difference is in testing. 2H nuts require hardness test on samples based on lot sizes. However 2HM nuts shall be 100% tested for hardness. For the testing of 2HM nuts ASTM 194 also specifies that the testing shall be carried out by considering 3 additional things as follows:
  •  ✔ Protect legibility of markings
     ✔ Minimize exterior dimensional changes
     ✔ Maintain thread fit.
  • As discussed above the differences are also in the hardness values.
  • Grade Completed Nuts Sample Nut after Heat-treat
    Brinell Hardness Rockwell Hardness,
    C Scale
    Rockwell Hardness,
    B Scale
    Brinell Hardness, min Rockwell Hardness B Scale, min
    2H through 1 ½" 248 to 327 24 to 35 - 179 89
    2H over 1 ½" 212 to 327 35 max 95 min 147 79
    2HM 159 to 235 - 84 to 99 159 84
  • Minimum tempering temperature for 2H is 455 ⁰C and that for 2HM is 620 ⁰C.



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