Heat treatment lab report politeknik

Heat Treatment and Hardenability of Steels

Abstract

This experiment is attempted to measure the hardenability of the steel and understand the process of heat treatment of different materials at different cooling strategies. Cooling through different procedures will cause the materials to have different properties and different microstructures. Furthermore next stage of experiment relates the cooling rate and hardness of 1045 steel and 4143 steel. This also helps in determine how alloying a material permits it to be heat treated more homogeneously. Investigated results also proven to be close enough to expected results in obtaining higher brittleness with rapid cooling in and to improve ductility the process of tempering is proven to be very efficient with increase of tempering temperature the hardness of material must be decrease. Last but not least, after finishing experiment 6 the group found out that the lower the tempering temperature the lower the hardness. Also, the results that the group found from experiment 7 after finishing it proved being inconsistent from what it should be.

Introduction

The purpose of this experiment is to determine what effect heat treating and then cooling has on the hardness and grain structure of two different types of steel. The two different types of steels were utilized are 1045 steel samples and 4143 steel sample which is considered to be a low-alloy steel.

The heat-treating process is a method to alter physical and mechanical properties of the material. The heat-treating process is consists of three crucial steps of annealing, hardening, and tempering. Annealing is primarily used to soften and to induce the ductility of the specimens by heating and holding at suitable temperature and then cooling, by instantly quenching in the water, which produces the higher brittleness with low ductility and toughness in the specimens. Moreover, tempering is a process of heat-treating, which is used to increase the toughness of metal. Tempering is important because it used to achieve desired hardness. To restore some the toughness and impact properties is obtained by tempering where specimens are reheated to a temperature between 5000 F and 10000 F for certain time which removes the internal strain caused by sudden cooling in the quenching bath without a large decrease in hardness or strength.

In attempting the first phase of the experiment it cannot determined why some heat-treated materials don’t reach a high hardness when cooled at certain temperature. With the hardness test the hardness of a material can be determined. The Hardenability is a property that determines the depth and distribution of hardness when steel is heated to a given temperature and then quenched to reach martensitic structure, which is obtained by performing Jominy test, where an austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the bar.

Procedure

First of all, the experiment provided six 1045 steel specimens were for heat treating process, and for the second were only two steel rods of 1045 steel and 4143 steel respectively used to perform the Jominy test. In order to go though the details read the following: First the group begin with identifying all each specimen by punching different letter on to them using hammer. Second, the engineer students heated all specimens at 16000 F for 1/2 hour after obtaining the Rockwell a scale hardness measurement. Third, The four samples were quenched in water, one sample is allowed for air cool, and the other sample is set for furnace cool for one hour and quenched in water. Then, the two steel rods of different properties also allowed for heated at 16000 F for 45 minutes after obtaining the Rockwell scale hardness measurements. Also, the group measured the Rockwell a scale hardness on all six quenched specimens after being heat treated and tempered the three-quenched specimen at different temperature of 6000 F, 8000 F, and 10000 F respectively for 30 minutes. After tempering specimens then quenched in water. Moreover, obtained hardness measurements using Brinell (3000 kg) and Rockwell A scale on all six steel specimens. In order to perform the Jominy test one steel rod is then removed from the furnace and is placed in the cooling tower for 10 minutes before quenching in water, repeated the same procedure for other steel rod. Finally, measured the hardness 1/16 inch for the first inch and every 1/8 for the next inch and 1/4 for the next 2 inches using Rockwell a scale for both steel rods.

Results and Discussions

The experiments “Heat Treatment of Steel” and “Hardenability of Steel” are two different experiments, which show the effects of heat-treating, and quenching of specimen provides different hardness and microstructure in the materials. During first phase of experiment the two specimens are left to cool at room temperature and furnace temperature, these specimens were quenched after an hour. The reason for this quenching after an hour is due to the fact that the grains in the material are given a chance to form when cooling at room temperature and furnace cooling temperature. If the grains are not given enough time to form when cooling at room and oven temperature the grain structure would not be accurate as if actually air cooled and furnace cooled. From the Table 1 it can be clearly seen the hardness obtained through furnace cooled is lesser than hardness obtained by air cooled specimens because in furnace cooling allow the grains to from due to its slow cooling process where as during air cool specimens tends to cool much quicker compare to furnace cool and specimens have less time to form grains. Due to that specimens will have more boundaries, which mean there will be more interference with dislocation motion. Also, in Table 1 it shows the Rockwell measurement is 76.99 for the instant quench. On the other hand, the furnace cooled is 55.05. Moreover, in Table 2 represent the Ultimate Tensile strength (psi) for all samples from the average Brinell Hardness number obtained. In fact, the hardness of both of the measured BNH and the measured Rockwell are decreasing. The Ultimate Tensile Strength (psi) is also decreasing because the hardness is going down. As we know the harder a material is the higher the strength is. Furthermore, the instant quenched sample has the highest hardness and the Ultimate Tensile strength results. Finally, Table 3 represents the hardness of the Steel 1045 sample after it has been placed at different tempering temperatures. Moreover, Table 4 shows the difference in hardness between Steel 1045 and Steel 4143 that that was taken at different distance from the quenched end.

Table 1 Comparison between performed Brinell hardness numbers measurements with Brinell hardness numbers obtained by conversion of Rockwell A scale measurements.

Specimens

Rockwell

A scale

measurements

RHA Conversion to BHN

Dimple Diameters (mm)

BHN from Dimple Diameters

S instant quench

76.99

500

2.50

601

H Tempered @540 0C

71.58

390

3.20

363

D Tempered @ 430 0C

69.19

353

2.90

444

K Tempered @ 315 0C

70.89

381

2.81

417

M (air cooled)

53.39

172

4.29

197

E (Furnace cooled)

53.05

169

4.51

179

Table 2 Computed Ultimate Tensile strength (psi) based on the average Brinell Hardness number obtained.

Specimens

Measured BHN

(3000)kg

Measured Rockwell A scale numbers (HRA )

Conversion BHN

Average BHN

Ultimate Tensile Strength (psi)

S

401

75.53

500

550.5

2.75E+05

D

429

71.71

353

398.5

1.99E+05

K

444

66.89

381

399

2.00E+05

H

388

66.99

390

376.5

1.88E+05

M

211

55.92

172

184.5

9.23E+04

E

363

52.10

169

174

8.70E+04

The obtained Brinell hardness comparing to Brinell hardness obtained from the conversion scale of Rockwell A scale hardness both results increase and decrease accordingly to the hardness. The data represented in Table 1 and Figure 1 shows that Brinell hardness increase in relation to the cooling rate and heat treating hardness for Rockwell A scale hardness measurements, but did not increase between specimens “K” and “D” instead hardness went down. If both the Brinell hardness and Rockwell a numbers were proportional to each other a straight line would be seen. A graph like the one shown in Figure 1 could be a result of an inaccurate machine or inaccurate measurement taking strategies.

Figure 1 Brinell Hardness numbers vs. Rockwell A scale numbers obtained after heat treating of the specimens.

Table 3 The hardness measurements obtained using Rockwell A scale for three 1045 steel specimens that has been tempered at different temperatures after being heat treated.

Specimens

Hardness Rockwell

A scale (HRA)

Tempering temperature ( c )

D

71.71

430

K

66.89

315

H

66.99

540

Figure 2 Hardness obtained using Rockwell A scale hardness after tempering the specimens.

To obtain desired mechanical properties in steel specimens it is necessary to process heat treating, quenching, and tempering of the steel. Hardening is way of making steel harder, by first heat treating the specimens to 8850 C for half hour and immediately cools it by quenching the specimens in water, which increase the brittleness of the substance at much higher rate with very low ductility and toughness in the samples. The tempering is the process through which brittleness is reduced to improve ductility and toughness by heating the specimens at different temperature for certain time.

Higher tempering temperature will yield a somewhat softer material with higher toughness, whereas a lower tempering temperature will produce a harder and somewhat more brittle material, as shown by the Figure 2 where hardness increases with the increase of tempering temperature.

The possible errors of not quenching the specimens in desired time or factors of obtaining the hardness of the samples at softer spot may have occurred in processing the tempering of specimens, which resulted on the graphs for not obtaining consistency.

The decline in hardness of tempered specimens once has been heat treated and quenched in Table 3 proves the hypothesis of decrease in brittleness by tempering the specimens.

The Ultimate tensile strength of materials is determined using equation 1 by using data collected for Brinell hardness for all the specimens mentioned in Table 2.

Equation 1 Calculating the Ultimate Tensile Strength of materials.

In determining the strength, obtaining the hardness is great ways of making comparison, which can be attain using Rockwell A scale and Brinell hardness scale which is directly proportional to the tensile strength. In using Brinell hardness scale timing in maintaining the load on the specimens may have been a factor of slight variation of results where as in Rockwell A scale ha

From the Figure 4 the amount of carbide ((Fe3C) can be calculated at temperature 1338 0F for 1045 steel using equation 2 where C1 is 45% because that is the weight percent of carbon in 1045 steel. Ca and Cb calculated using the lever rule which consists of drawing a line across to determine how much weight percent of material there is in the steel, where “a” is alpha and “b” is Iron Carbide (Fe3C). Using equation 2 is determined that Iron carbide percent is about 0.68% and 99.32% is presumed to be alpha phase.

Wb = (C1 – Ca)/(Cb – Ca) , Ca= 2.2%, and Cb= 65% are the weight percent composition.

Equation 2 To find the Fe3C (carbide) content using weight percent equation.

Figure 4 The iron-iron carbide phase diagram.

Different microstructures obtained when specimens processed through differen cooling strategies that is why the TTT (Time, Temperature, Transformation) chart in Figure 5 is proven to be great tool in determining the microstructure. The TTT chart shows the amount of time needed to quench a material to reach a certain phase. The left part line represents the beginning of the transformation and the right part line represents the conclusion of the transformation. The TTT chart also explains the need of quenching the specimens after an hour of cooling due to after certain time the specimens does not require any more transformation. The martensite structure which is one of the hardest of all phases is obtained upon quenching instantly to a low temperature. But the other samples that quenched were temperd again to move higher up in the TTT chart where less hardened materials are . To obtained a desired phase it is neccsary to for rapid change in temperature with respect to time when quenching the 1045 steel to reach a Bainite phase.

The specimens that were furnace cooled and cooled at room temperature are most likely to fall in the pearlite phase where the one cooled at room temperature is said to be fine pearlite while the other one is more close to coarse pearlite due to slow cooling process. The specimens tempered at 540°C falls between pearlite and bainite phases. The specimens tempered at 430°C and 315°C fall under the Bainite phase, the one tempered at a lower temperature could be classified as being of finer Bainite.

Figure 5 The TTT (Time, Temperature, Transformation) chart for 1045 steel.

Microstructure and hardness are closely correlated; microstructure consists of grain size and crystal structure. When the specimens were reached at austenite phase, the grains are more uniform and homogeneously distributed; upon completion of this process the specimens are ready to be cooled in order to obtain different hardeneability in the material. During instant quenching of the specimens the specimens with evenly distributed grains are not given a chance to form and are then solidified giving the material a fine grain structure in contrast to a material slowly cooling which gives a material more coarse grains making the material less hard and more ductile. The Jominy test results illustrated in Figure 3 prove that how cooling rate affects hardness data obtained on the attempt of experiment. The greater distance of quenching the less hard the material is because, as mentioned before, the grains are given more time to form, and the bigger the grains the less hard the material. The inconsistency among results obtained instead of constantly moving downward may have caused due to experimental errors such as not placing it fast enough on the Jominy tester. The graph line obtained by the 4143steel and 1045 steel quickly goes up and down not opening the water enough for quenching during the Jominy test. The overall graph is also does not matches to the expected results where 1045 steel graph must lower than the graph line obtained by the 4061 steel rod specimen is considered to be an experimental error of not transporting the specimen on the tester with in time duration.

Table 4 The Jominy test results obtained on two steel rods.

Distance from quenched end (in)

1045 Steel

4143 Steel

0.0625

75.9

66.2

0.125

72.2

66.4

0.1875

61.7

66.5

0.25

50.3

68.5

0.3125

58.2

67.6

0.375

56.9

61.3

0.4375

55.6

65.1

0.5

55.9

60.8

0.5625

53.6

60.6

0.625

55.7

61.4

0.6875

54.4

57.8

0.75

49.7

58.2

0.8125

51.8

53.8

0.875

51.2

55.9

0.9375

51.6

51.9

1

50.2

56.6

1.125

49.2

52.3

1.25

50.8

55

1.375

50.5

53.5

1.5

48.4

51.8

1.625

49.2

49.8

1.75

49.1

51.9

1.875

47.8

52

2

47.2

47.7

2.25

47.3

49.7

2.5

47.5

49.7

2.75

46.9

45.5

3

44.2

45.8

3.25

38.6

45.1

3.5

40.1

22.9

3.75

42.7

32

4

36.4

26.3

Figure 3 Plot showing Hardness as a function of distance from the quenched end for 1045 steel and 4061 steel specimens. The dash line is the Steel 4061 and the solid line is the Steel 1045.

Conclusion

Materials that cool at slower cooling rates tend to be softer materials while those that are cooled at faster cooling rates tend to be harder. Tempering a material lowers its ultimate strength but increases the amount of stress the material can absorb (toughness), higher the tempering temperature the lower the ultimate strength. Tempering also adds more ductile characteristics to the material. High hardness in materials only can be attain when there is a low toughness, in order to acquire toughness in a material that has been quenched, The tempering of the specimens is then processed to improve the toughness in the material and lower the brittleness. Fine grain structures tend to be hard material where as materials with coarse grain structure has more ductile properties. Furthermore, the data does not accurately show what should be happening. The data I collected has error that is obviously shows in my graph. The graph should show a straight line going down similar to the one in experiment 6. For example, after heating the Steel and quenched it the group had to measured it and probably by mistake the engineer student measured the same point twice or took similar points close to each other. Also, maybe the time it took to move the Steel from furnace to be quenched was too long which effected the measurements. Moreover, the water that was used to quench could have been too strong hitting the specimen, which leads to make a huge differences in measurements.

References

D. Callister Jr, Fundamentals of Materials Science and Engineering, J. Wiley & Sons, NY, 3rd Ed. 2008, Flinn and Trojan, Engineering Materials and Their Application, Chapter 6 Dieter, Mechanical Metallurgy ASM Handbook on Heat Treatment, Vol. 2

http://www.smt.sandvik.com/en/products/strip-steel-and-strip-based-products/strip-products/knife-steel/hardening-guide/purpose-of-hardening-and-tempering/

http://www.carbidedepot.com/formulas-hardness.htm

William D. Callister, Jr., David G. Rethwisch. Fundamentals of materials science and engineering, third edition

www.csun.edu/~bavarian/Courses/MSE%20227/Labs/2-Charpy_test.pdf

Distance From Quenching (in) vs. Hardness (HRA)

1045 Steel 0.0625 0.125 0.1875 0.25 0.3125 0.375 0.4375 0.5 0.5625 0.625 0.6875 0.75 0.8125 0.875 0.9375 1.0 1.125 1.25 1.375 1.5 1.625 1.75 1.875 2.0 2.25 2.5 2.75 3.0 3.25 3.5 3.75 4.0 75.9 72.2 61.7 50.3 58.2 56.9 55.6 55.9 53.6 55.7 54.4 49.7 51.8 51.2 51.6 50.2 49.2 50.8 50.5 48.4 49.2 49.1 47.8 47.2 47.3 47.5 46.9 44.2 38.6 40.1 42.7 36.4 4061 Steel 0.0625 0.125 0.1875 0.25 0.3125 0.375 0.4375 0.5 0.5625 0.625 0.6875 0.75 0.8125 0.875 0.9375 1.0 1.125 1.25 1.375 1.5 1.625 1.75 1.875 2.0 2.25 2.5 2.75 3.0 3.25 3.5 3.75 4.0 66.2 66.4 66.5 68.5 67.6 61.3 65.1 60.8 60.6 61.4 57.8 58.2 53.8 55.9 51.9 56.6 52.3 55.0 53.5 51.8 49.8 51.9 52.0 47.7 49.7 49.7 45.5 45.8 45.1 22.9 32.0 26.3
Distance From Quenching (in)

Hardness (HRA)

Brinell Hardness (krg) vs. Rockwell A Hardness (hra)

BHN vs. Rockwell A 601.0 444.0 417.0 363.0 197.0 179.0 76.99 69.19 70.89 71.58 53.39 53.05
Brinell Hardness (krg)

Hardness (Rockwell A scale) hra

Rockwell A Hardness vs. Tempering Temperature 540.0 315.0 71.58 70.89
Tempering Temperature (C)