6.21- The data below were obtained from a series of Charpy impact tests performed on four steels, each having a different manganese content. Plot the data and determine (a) thetransition temperature (defined by the mean of the absorbed energies in the ductile and brittle regions) and (b) the transition temperature (defined as the temperature that provides 50 Jabsorbed energy). Plot the transition temperature versus manganese content and discuss the effect of manganese on the toughness of steel. What would be the minimum manganese allowedin the steel if a part is to be used at 0oC?
(a) Transition temperatures defined by the mean of the absorbed energies are: 0.30% Mn: mean energy = 2 + (130 + 2)/2 = 68 J; T =27oC 0.39% Mn: mean energy = 5 + (135 + 5)/2 = 75 J; T = 10oC 1.01% Mn: mean energy = 5 + (135 + 5)/2 = 75 J; T = 0oC 1.55% Mn: mean energy = 15 + (140 + 15)/2 = 92.5 J; T = −12oC (b)Transition temperatures defined by 50 J are: 0.30% Mn: T = 15oC 0.39% Mn: T = −5oC 1.01% Mn: T = −15oC 1.55% Mn: T = −45oC
Increasing the manganese increases the toughness andreduces the transition temperature; manganese is therefore a desirable alloying element for improving the impact properties of the steel. If the part is to be used at 25oC, we wouldwant at least 1.0% Mn in the steel based on the mean absorbed energy criterion or 0.36% Mn based on the 50 J criterion. 7.2- A 1.5-cm-diameter metal bar with a 3-cm gage length issubjected to a tensile test. The following measurements are made.
Determine the strain hardening coefficient for the metal. Is the metal most likely to be FCC, BCC, or HCP? Explain.t = Ktn
ln 143 = ln K + n ln 0.2 ln 249 = ln K + n ln 0.6 (4.962 − 5.517) = n(−1.609 + 0.511)
n = 0.51 A strain hardening coefficient of 0.51 is typical of FCC metals.
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