Steel hardness vs yield or ultimate strength?

[JAG-E]

I have found sources that state hardness can't be correlated to yield strength, but only to ultimate strength. Other sources state the opposite. Yet other sources claim you can do both with different equations.

Can someone direct me to a source that addresses my question, or addresses the divergent opinions? Is it that some test methods can do one or the other; Sy vs Su. The source that states hardness can't be correlated to yield is a text from 1941. New developments/methods since then?

Thanks,
Joe

 

[Mech_Engineer]

From what I understand steel's hardness is fairly well-correlated with ultimate tensile strength of the material. ASTM A370 provides the standard test methods and even a table which shows correlations between hardness in several different scales and the steel's tensile strength.

This website provides a table which they say is based on the ASTM A370 standard, and shows correlations between tensile strength and hardness.

For a quick sanity check, I looked up a few common steel alloys to see if the table works:

• A36 Structural Steel:
  • Ultimate Strength: ~68 ksi (avg of min and max reported by MatWeb)
  • Hardness (Table Correlation): Brinell 137
  • Hardness (Reported by Cardinal Metals): Brinell 137

• 1045 Carbon Steel:

• Ultimate Strength: 82 ksi (reported by MatWeb)

• Hardness (Table Correlation): Rockwell B 85

• Hardness (reported by MatWeb): Rockwell B 84

• Stainless Steel 17-4ph:
  • Ultimate Strength: 175 ksi (reported by MatWeb)
  • Hardness (Table Correlation): Rockwell C 39
  • Hardness (reported by MatWeb): Rockwell C 39

• Stainless Steel 440C:
  • Ultimate Strength: 254 ksi (reported by MatWeb)
  • Hardness (Table Correlation): Rockwell C 49
  • Hardness (reported by MatWeb): Rockwell C 58

So overall the correlation seems to be ballpark reasonably accurate for carbon steels like A36 and AISI 1045 and even Stainless 17-4ph, but there is a significant error for Stainless 440C. It's my recollection this form of correlation is used to non-destructively estimate the tensile strength of a part by performing a hardness test on it. It can give you a ballpark number without having to test the part to its breaking point in a tensile test rig.

 

[JAG-E]

From what I understand steel's hardness is fairly well-correlated with ultimate tensile strength of the material. ASTM A370 provides the standard test methods and even a table which shows correlations between hardness in several different scales and the steel's tensile strength.

This website provides a table which they say is based on the ASTM A370 standard, and shows correlations between tensile strength and hardness.

View attachment 225690

For a quick sanity check, I looked up a few common steel alloys to see if the table works:

• A36 Structural Steel:
  • Ultimate Strength: ~68 ksi (avg of min and max reported by MatWeb)
  • Hardness (Table Correlation): Brinell 137
  • Hardness (Reported by Cardinal Metals): Brinell 137

• 1045 Carbon Steel:

• Ultimate Strength: 82 ksi (reported by MatWeb)

• Hardness (Table Correlation): Rockwell B 85

• Hardness (reported by MatWeb): Rockwell B 84

• Stainless Steel 17-4ph:
  • Ultimate Strength: 175 ksi (reported by MatWeb)
  • Hardness (Table Correlation): Rockwell C 39
  • Hardness (reported by MatWeb): Rockwell C 39

• Stainless Steel 440C:
  • Ultimate Strength: 254 ksi (reported by MatWeb)
  • Hardness (Table Correlation): Rockwell C 49
  • Hardness (reported by MatWeb): Rockwell C 58

So overall the correlation seems to be ballpark reasonably accurate for carbon steels like A36 and AISI 1045 and even Stainless 17-4ph, but there is a significant error for Stainless 440C. It's my recollection this form of correlation is used to non-destructively estimate the tensile strength of a part by performing a hardness test on it. It can give you a ballpark number without having to test the part to its breaking point in a tensile test rig.

Thanks for the info. The hour is late so I will hit this tomorrow. One problem is the vocabulary. Without much thought of the use of the word tensile, I always took that to mean ultimate. When we were addressing yield, yield was explicitly stated. Articles will speak of hardness correlating to ultimate and then have a chart similar to the one in the link you provided, but list tensile not ultimate at the top of the column. Webster defines tensile as breaking apart which would be ultimate. Wiki gets a little loose, but may be reflecting the loose way the word is used:

(From Wiki) "There are three typical definitions of tensile strength:

• Yield strength - The stress a material can withstand without permanent deformation. This is not a sharply defined point. Yield strength is the stress which will cause a permanent deformation of 0.2% of the original dimension.

• Ultimate strength - The maximum stress a material can withstand.

• Breaking strength - The stress coordinate on the stress-strain curve at the point of rupture."

I think I have this problem now with a professor who thinks tensile means pulling and believes hardness is a correlation to yield.

Thanks,
Joe

 

[JBA]

In general application and metallurgical terms "tensile strength" and "ultimate strength" are used interchangeably to refer to the failure point of a material under a tensile (tension) loading as opposed to a compression loading.
While the term "tensile strength" has its own individual Webster dictionary listing and definition as: "the greatest longitudinal stress a substance can bear without tearing apart", there is no corresponding Webster dictionary listing and definition for the term "ultimate strength".

 

[JAG-E]

" "tensile strength" and "ultimate strength" are used interchangeably to refer to the failure point of a material under a tensile (tension), that is what I alway understood. I wanted to cross check my understanding. Today I spoke to a guy who ran the test lab at US Steel. He agrees with "tensile strength" and "ultimate strength" are used interchangeably. Also that hardness relates best to Ultimate vs yield. I was surprised to find opinions to the contrary.

Thanks,
Joe

 

[Mech_Engineer]

ASTM A370 defines tensile strength below, this is the definition used in the correlation table provided above:

Tensile Strength—Calculate the tensile strength by dividing the maximum load the specimen sustains during a tension test by the original cross-sectional area of the specimen.

ASTM A370 also provides the following regarding the hardness/tensile strength correlation:

This table gives the approximate interrelationships of hardness values and approximate tensile strength of steels. It is possible that steels of various compositions and processing histories will deviate in hardness-tensile strength relationship from the data presented in this table. The data in this table should not be used for austenitic stainless steels, but have been shown to be applicable for ferritic and martensitic stainless steels.