Fracture Toughness of High-Strength Steels at Low Temperatures

Current and developing applications for materials at low temperatures include structures, vehicles, and pipeline equipment for arctic environments, storage and transport equipment for liquefied fuel gasses, oxygen and nitrogen, and superconducting machinery, devices and electrical transmission systems. Most of these applications relate to the production and distribution of energy and have attained greater prominence because of the current energy shortage.

According to available information on the fracture toughness of high-strength alloys at low temperatures, the effect of low temperatures on toughness is generally dependent on the alloy base. For many aluminum alloys, the fracture toughness tends to increase or remain generally constant as the testing temperature is decreased. Titanium alloys tend to have lower toughness as the testing temperature is decreased, but the effect is influenced by the alloy content and heat treatment. Certain titanium alloys retain good toughness at very low temperatures. Alloy steels normally exhibit decreasing fracture toughness as the testing temperature is decreased through transition temperature range, when the structure contains ferrite or tempered martensite. The transition temperature is influenced by the alloy content, grain size and heat treatment.

Carbon and low alloy steels represent body-center-cubic (bcc) atomic lattices and exhibit toughness transition temperature ranges either above, at, or below room temperature depending on a number of factors. At temperatures above the transition temperature, the alloy has substantially better toughness than at lower temperatures. Furthermore, the lower strength steels generally are strain-rate sensitive, while the higher strength steels are not strain-rate sensitive

The curves for parent metal and welds in ASTM A517F steel plate indicate that the weld metal and heat-affected zones have lower transition temperatures than the parent metal. However, the weld metal in the specimens of A542 steel had higher transition temperatures than the parent metal. The fracture toughness of A533 Grade B Class 1 steel has been studied extensively for nuclear reactor pressure vessels. This study has shown that for testing temperatures above -100^oF (approx. -70^oC), the toughness increases substantially as the testing temperature is increased.

Thus the required thickness of the specimens must be increased in order to increase the constraint that is necessary at the crack tip to simulate plane-strain conditions at the initiation of fracture.

Results of fracture toughness tests on three ASTM forging steels may have similar general trends in the toughness data, but the compositions, grain sizes, and other factors have marked effects on the transition temperatures. Test results for HY-130 steel indicate that this steel in the temperature range down to -320^oF (approx. -195^oC) is not strain-rate sensitive.

These steels are not intended for use at temperatures in or below the transition temperature range, and there is no accepted method for indicating the specific transition temperature from a transition temperature curve. Furthermore, there is no accepted method relating the transition temperature to a safe minimum service temperature for structural components. However, if Kic data are obtained for given alloy at low temperatures, the critical crack sizes may be estimated in the low-temperature range at the maximum service stress of the structure.

The effects of variations in composition on a series of Ni-Cr-Mo-V steels has been studied in order to show the effects of the alloying elements on the low-temperature fracture toughness. Bars of these steels were quenched and tempered to about 170 ksi yield strength (approx. 1170 MPa) and tested as precracked bend specimens. The effects of carbon and nickel content were the most significant. An increase of carbon content and nickel content from 0.28 to 0.41 raised the transition temperature based on KQ data. Increasing the nickel content from 1.26 to 6.23 percent decreased the KQ transition temperature. This represents one of the major attributes of nickel additions to the alloy steels.

The specimens of D6ac steel were austenitized at about 1650^oF, furnace cooled to 975^oF, and quenched in oil or molten salt according to several different procedures, to simulate quenching of the welded forging that comprise the F111 wing cary-through structure. The high-toughness specimens were quenched in oil, while the medium-toughness specimens were quenched in salt. Regardless of the quench, the yield strength of the specimens was approximately 217 ksi (1495 MPa) after tempering twice at 1000 to 1025^oF. The fracture roughness tests were very sensitive indicators of the effect of the variation in quenching rate on the toughness. The specimens that had the highest toughness at room temperature also had the highest toughness at -65^oF (-54^oC).

Available fracture toughness data at low temperature for other alloy steels: AISI 4340, 300M, HP9-4-20, HP9-4-25, and 18 Ni (200) maraging steel usually have the trend of decreasing toughness as the testing temperature is decreased. The one exception is HP9-4-25 in the temperature range +75 to -75^oF (+24 to -59^oC). At lower temperature, the expected trend would be for the toughness to drop as indicated for HP9-4-20 in the range from -100 to -320^oF (-73 to -195^oC). The data obtained by Steigerwald for AISI 4340 steel and by Wessel for the HP9-4-20 alloy steel were obtained before ASTM Method E 399 was available and are designated as KQ values.

The 18Ni (200) grade maraging steel also exhibits considerable reduction in toughness as the testing temperature is reduced from -100 to -320^oF (-73 to -195^oC), but at -320^oF, this heat of the 200 grade retained a toughness of about 80 ksi in.1/2. From limited information on the toughness of the 200 grade, it appears that there is considerable range in results of KIc tests at room temperature. This level of toughness at -320^oF probably can be achieved only if the alloy has a toughness of about 160 ksi in.1/2 or over at 75^oF (+24^oC).

The effect of low temperatures on the static and dynamic fracture toughness of bend specimens of 18Ni (200) maraging steel is a straight line relationship between the Kic values and the testing temperature in the range from 75 to -320^oF. At -320^oF, the KIc value was about 40 ksi in.1/2, and the alloy is not strain-rate sensitive in the low-temperature range. Tests results on part-through surface-crack specimens of 200 grade maraging steel has shown that these heats had high toughness at 75^oF and also retained relatively good toughness at -320^oF. With optimum welding conditions, the weld metal also retains good strength and toughness at -320^oF.