Transmission Electron Microscopic Observation of Low Carbon Steels During Creep

Abstract

It is well known that the creep strength of low carbon steels is affected by N, Mn and Si contents as well as heat treatment. But the mechanism by which these factors affect the creep strength of carbon steels is still somewhat unclarified.
The authors made creep rupture tests of low carbon steels with various chemical compositions to study the effect of Al, N and Mn on the creep strength of low carbon steels, and observed the structural changes during the creep by transmission electron microscopy.
The behaviour of N during creep was also investigated with the internal friction method and the mechanism by which Al, N and Mn affect the creep strength of low carbon steels was discussed.
Results are summarized as follows:
(1) Creep rupture strength:
(a) Effect of Al:
Al addition drastically reduces the creep rupture strength of low carbon steels, but Al not exceeding 0.015% lowered it only moderately. An Al-killed steel with 0.039% Al had the lowest creep rupture strength.
(b) Effect of N:
A vacuum melted steel with 0.002% N showed the lowest strength only second to the Al-killed steel.
The higher the N contents were increased the higher the creep rupture strength rose, but the addition of N exceeding 0.005% had relatively small effect.
(c) Effect of Mn:
The increase in Mn contents from 0.5% to 1.2% caused small decrease in the creep rupture strength.
(d) Effect of Mn-N:
When both Mn and N contents simultaneously increased, the creep rupture strength was greatly improved. A steel with 1.23% Mn and 0.02% N showed the highest creep rupture strength among the steels investigated.
(2) Transmission electron microscopy:
(a) Vacuum melted, low N steel:
The cell structure was formed during the creep deformation. The progress of the cell formation was different from one grain to another, from ones in which cells were formed in the primary stage to the others in which cells were not formed until the tertiary stage. The number of grains in which cells were formed increased with the increase in creep strain, and when the specimen ruptured, cells were formed in all the grains.
The cell size decreased to about μ in the primary stage and almost unchanged during the secondary stage, followed by small decrease in the tertiary stage.
The dislocation density within a cell was nearly constant during the secondary stage and rapidly increase with the steep increase in strain during the tertiary stage. The dislocation density within a cell was lower than that within a grain in which cells were not formed, suggesting that some dislocations were annihilated at the cell boundaries during the cell formation.
(b) Al-killed steel:
Fine, coherent precipitates, about 150Å in size and presumedly AlN, were observed in a normalized state. Although, at room temperature, the precipitates seemed to contribute to the strength of this steel, during creep at elevated temperatures, the precipitates were rapidly coursened and seemed to have no contribution to the creep strength.
The precipitation decreased in the amount of N in solid solution and badly reduced the creep strength.
(c) High Mn-high N steel:
The cell structure was not observed even when the specimen ruptured.
Fine, coherent precipitates, about 100Å in size, were observed after the early half of the secondary stage of creep and also after about 800 hours of aging at 450°C.
Similar precipitates were observed in a low Mn-high N steel and a high Al-high N steel.
(3) Internal Friction measurements:
Snoek damping of the high Mn-high N steel was measured using about 400cps vibration during creep.
During the primary stage, while the creep rate decreased, the Snoek peak was as high as the initial value, suggesting that almost all of N remained in solid solution.
The Snoek peak began to decrease in the early half of the secondary stage, not accompanied with any marked change in the creep rate.