FIELD OF THE INVENTION
The present invention relates to method for Determining Ductile-Brittle-Transition-Temperature (DBTT) for Cryogenic Steel.
BACKGROUND OF THE INVENTION
Toughness is the ability of materials to absorb energy during the process of plastic deformation and fracture, and is the display of strength and plasticity. It is of great importance in engineering to conduct a series of impact tests on cryogenic steels at different temperatures to determine the brittle tendency of these materials at falling ambient temperatures. When the ambient temperature drops, the toughness of materials also decrease and it becomes very low at a certain low temperature. This is called cold brittleness, and the temperature at which the material turns from a tough state to a brittle state is called the ductile-brittle transition temperature (DBTT).
Carbon and alloy grades of steel for applications at cryogenic temperatures are required to provide high strength, ductility, and toughness in vessels, and structures that are exposed to temperature -45°C and lower. Since a number of steels are engineered exclusively for applications at low temperature (about -100°C), deciding the optimum material requires detailed understanding of the application and knowledge of the mechanical properties that each grade provides. Crystal structure of metals plays an important role in deciding their characteristics at below ambient temperature. The yield and tensile strengths of metals that have body-centered cubic crystal structure such as iron, molybdenum, vanadium and chromium depend greatly on temperature.
These metals exhibit a loss of ductility in a narrow temperature region below room temperature. The best suitable alloy steel for cryogenic temperatures (down to -195°C) is 9% nickel steel. It is used for transport and storage of cryogenic liquids because of its low cost and ease of fabrication. Other alloy

steels such as A201 and T-l can be used up to -45°C, while nickel steels with 2.25% Ni can go up to -59°C, and nickel steels with 3.5% Ni up to -101°C.
For any cryogenic assembly, the highest temperature the material will encounter is room temperature. Therefore, it is appropriate to do stress calculations on the room temperature properties of the material.
Conventional Charpy impact tests for chemistry as (wt%): C-0.09, Mn-1.55, S-0.01, P-0.011, Si-0.240, Nb-0.028, Ti-0.015, B-20(ppm), N-72(ppm), rest Fe reveal that the DBTT is -75°C (shelf energy ~27Joules) for the designed and in house fabricated steel.
FIG. 1 shows the Charpy impact energy values as a function of temperature. It can be seen that DBTT temperature for the investigated grade is around -75°C (27 Joules shelf energy value). However, it needs to be noted that DBTT value obtained from low temperature tension tests is way below the Charpy transition temperature. The reason for inaccurate DBTT can be the strain rate in Charpy tests is extremely high Also due to the presence of notch, the stress
intensity factor is extremely high thereby causing cleavage fracture in Charpy tests. This can be supported by the fact that for notched tensile tests, the material fails in a cleavage manner both at room temperature and at very low temperature. Also it needs to be noted that for tough pearlitic rail steel, the Charpy energy value is of the order of 11 joules at room temperature.
Actually, Charpy impact test is not a true indicator of plasticity for materials. It is known that with a suitable stress state a ductile material can be made brittle and the rate also has a profound effect on plasticity since it has been demonstrated in many studies especially in automobile crash resistance study that high strain rate causes the steel to fail in a brittle manner. Also it is known that in tension tests, the pearlitic steel wire rod shows moderate ductility but the same wire rod can be drawn to 4-5 true strain during wire drawing manufacturing operations.

OBJECTS OF THE INVENTION
In view of the foregoing limitations inherent in the prior-art, it is an object of the invention to develop a new method for calculating DBTT accurately.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a method for determining ductile-brittle-transition-temperature (DBTT) of a steel sample. The method comprises steps of calculating fracture strength of a first notched steel sample at a first temperature, calculating yield strength of a second steel sample at the said first temperature, repeating step (a) and step (b) for considerable number of times for different temperatures and extrapolating fracture strength and yield strength over strength-temperature graph to determine DBTT of the steel sample.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 shows Charpy impact energy (Joules) values as function of temperature for steel sample in accordance with an embodiment of the invention.
FIG. 2 is flow diagram showing various steps for determining DBTT of steel sample in accordance with an embodiment of the invention.
FIG. 3 depicts yield strength and fracture strength as a function of temperature of steel sample in accordance with an embodiment of the invention.
FIG. 4 shows room temperature tensile stress strain diagram of steel sample, rolled alloyed, in accordance with an embodiment of the invention.
FIG. 5 shows Macro-photograph of the room temperature tested broken steel sample showing extensive necking and failure demonstrating ductility in accordance with an embodiment of the invention.

FIG. 6 shows graph showing stress-strain behavior of the smooth steel samples at various cryogenic temperature in accordance with an embodiment of the invention.
FIG. 7 shows load-displacement behavior of the notched steel sample in accordance with an embodiment of the invention.
FIG. 8 shows macrophotograph showing cleavage for notched steel sample and cup and cone fracture for un-notched steel samples at liquid nitrogen test temperature.
FIGS. 9(a)-9(b) shows fractograph showing cleavage fracture of notched steel sample at -120°C temperature and the dimpled fractured surface of the failed low temperature (-120°C) tested smooth steel sample respectively.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a method for determining ductile-brittle-transition-temperature (DBTT) of a steel sample, the method comprising steps of: step (a) calculating fracture strength of a first notched steel sample, at a first temperature step (b) calculating yield strength of a second steel sample at the said first temperature repeating step (a) and step (b) for considerable number of times for different temperatures and extrapolating fracture strength and yield strength over strength-temperature graph to determine DBTT of the steel sample.
In accordance with an embodiment of the invention steel sample whose Ductile Brittle Transition Temperature (DBTT) is to be found is an alloy made using induction furnace. To make steel sample, billet is cut into smaller pieces. Before forging, the billet is heated to 1250°C and soaked for couple of hours and forged to 66% reduction and later the plates are again soaked at 1250°C for couple of hours and rolling is completed above 850°C with a reduction of 40%. The final thickness of the rolled plates is 16 mm. Both after forging and rolling, the slabs

are left in the ambient air. The samples are analyzed just after heat making and subsequently after forging and rolling. The target chemistry is achieved and is given as (wt%): C-0.09, Mn-1.55, S-0.01, P-0.011, Si-0.240, Nb-0.028, Ti-0.015, B-20(ppm), N-72(ppm), rest Fe .
It should be noted that the sample can also be of any other grade of steel or alloys manufactured by other methods. The sample should contain the ferritic bainitic structure.
Shown in FIG. 2 is a flow diagram showing various steps for determining DBTT of the steel sample. Following are the steps:
Step (a): calculating fracture strength of a first notched steel sample, at a first temperature;
Step (b): calculating yield strength of a second steel sample at the said first temperature;
Step (c): repeating step (a) and step (b) for considerable number of times for different temperatures; and
Step (d): extrapolating fracture strength and yield strength over strength-temperature graph to determine DBTT of the steel sample.
The sample with notch and un-notched is tested at low temperatures as shown in FIG. 2. For the calculation of the fracture strength and the yield strength is conducted in a quasi-static mode, meaning the strain rate of the order of 10-3s-1 is applied. The first notched steel sample have 45 degree notch all around in the middle and the root radius of 0.25 mm is provided. The notch sensitivity ratio (NSR) is more than 1.
Step (d) can be graphically represented by FIG. 3. The graph is plotted between strength (in MPa) along Y-axis and Temperature (in °C) along X-axis. It can be seen that at each and every single temperature two data have been identified and plotted. The upper data is for fracture strength and the lower data is for yield strength.

It can be seen that there is variation of yield strength and fracture strength as a function of temperature at a slow rate. Yield strength is quite insensitive to change in temperature and the fracture strength is higher than the yield strength for the entire low temperature range and that supports the fact why the material has excellent cryogenic properties. Since the yield strength is lower than the fracture strength hence the material yields before it fractures. It is known that when fracture strength and yield strength coincides at a particular temperature that temperature is the true DBTT and it can be seen that can happen way below -165°C. The DBTT for the steel sample having composition by wt%: C-0.09, Mn-1.55, S-0.01, P-0.011, Si-0.240, Nb-0.028, Ti-0.015, B-20(ppm), N-72(ppm) rest Fe is -196oC.
Shown in FIG. 4 is the room temperature tensile stress-strain diagram of the sample, which is rolled alloy, showing excellent work hardening capability of the material. It can be seen that the yield strength is of the order of 420-430 MPa is observed while the UTS is around 680 MPA. Work hardening of the developed steel sample is excellent. Total elongation of the order of 22-25% is noted at room temperature tests.
Shown in FIG. 5 is macro-photograph of the room temperature tested on the steel sample which got broken, showing extensive necking and failure which demonstrates remarkable ductility of the designed alloy sample.
Various tension tests are conducted on un-notched steel sample, starting at -120°C at a slow rate and increased to higher temperature and this is shown in FIG. 6. It can also be seen that in all the samples more than 20 % total elongation have been obtained at low temperatures.
FIG. 7 shows the load-displacement behavior of notched steel samples at the indicated temperature. It should be noted that even the sharp notched specimen shows a lot of plasticity thereby demonstrating the sample alloy as notch tough at cryogenic temperature.

The purpose of conducting notch tensile test is to make the deformation highly constrained. This constrain is given with a sharp notch. As it is already known, when a smooth tensile specimen is pulled neck forms gradually and eventually triaxial state of stress sets in the specimen. With further deformation, triaxiality of stress state gets severe and the specimen fails abruptly. Now it has been shown that the triaxiality can be provided with a sharp notch so no plastic deformation occurs and that simulates quite closely to brittle fracture stress.
The load-displacement diagram (as shown in FIG. 7) of the notched tensile tested samples at two different temperatures. It can be seen from the diagram that there is appreciable plasticity even in notched steel samples and it is in the range of 1-2 % both at -120°C and at -100°C. It is evident that the sample in this study is tough even at -120°C and -100°C. The fracture strength is more than twice of the yield strength of smooth sample.
Stress triaxiality can be evaluated in terms of the triaxiality ratio
where the mean normal stress, and is the equivalent stress is
defined by

where are the principal stresses. For a typical uniaxial tensile test on
smooth sample, Von-Mises criteria tells us that is the yield stress of the materials meaning (yield strength). The same is true for Tresca
criterion as well. Bridgman showed that the triaxiality ratio at the center of a neck has the form of


where a is the radius of the minimum cross section, and R is the radius of curvature at the neck. Now in our case, plugging the values of a (2.5mm) and R (0.25mm), the equation 2 yields the triaxiality ratio and this translates to
and thus the notch provide a very high state of triaxiality and thus it
kmakes the sample fail abruptly in a brittle manner. The relation between a/R and plastic strain is represented by

where A is a constant and is the plastic strain at the onset of necking. Thus from the true stress strain diagram both and can be measured and therefore
for a specific situation, a/R also can be estimated which in turn gives the value of constant A. Once the constant A is known, the progression of triaxiality can be measured easily.
Shown in FIG. 8 is macro-photograph showing cleavage for notched steel sample and cup and cone fracture for unnotched steel samples at liquid nitrogen test temperature.
Shown in FIG. 9(a) is the cleavage fracture of notched steel sample at -120°C temperature. The SEM photomicrograph is of the failed notched steel sample and it can be seen that the fracture surface has a cleavage pattern which is very typical of brittle fracture. FIG. 9(b) shows the dimpled fractured surface of the failed low temperature (-120°C) tested smooth steel sample. This shows unnotched (smooth) tensile sample has a dimpled fracture surfaces and this is

the very essence of ductile mode of fracture and that too at -120 ° C temperatures.
The advantage of the invention is that it accurately measures the DBTT of the sample.

WE CLAIM:
1. A method for determining ductile-brittle-transition-temperature (DBTT) of a steel sample, the method comprising steps of:
step (a) calculating fracture strength of a first notched steel sample at a first temperature;
step (b) calculating yield strength of a second steel sample at the said first temperature;
repeating step (a) and step (b) for considerable number of times for different temperatures; and
extrapolating fracture strength and yield strength over strength-temperature graph to determine DBTT of the steel sample.
2. The method for determining ductile-brittle-transition-temperature (DBTT) as claimed in claim 1, wherein the notched steel sample is notched at 45 deg.
3. The method for determining ductile-brittle-transition-temperature (DBTT) as claimed in claim 1, wherein the notch sensitivity ratio is >1.
4. The method for determining ductile-brittle-transition-temperature (DBTT) as claimed in claim 1, wherein a strain rate for calculating the fracture strength and the yield strength is ~10-3 S-1.