Determination of cold brittleness temperature by means of serial dynamic bending tests

The serial dynamic bending (impact toughness) test of notched specimens is the most widespread determination method of temperature critical value (Т_CR).

This method is widely used at estimation of steel liability to brittleness while working at low temperatures. Low temperatures increase resistance of metals to plastic deformation and rise metals liability to brittle failure. The main aim of impact bending tests at low temperature is to define the minimal temperature at which the metal can work reliably with a small probability of brittle failure formation.

The information received as a in result of conducting of series of impact bending tests at the gradual decrease of FATT is used at serial curves construction. There is a sudden lowering of impact toughness at a certain temperature (temperature interval) called the critical brittleness temperature. The critical brittleness temperature is a comparative characteristic of metal liability to brittle failure at low temperatures.

The ductile-brittle transition in a short critical temperature interval appears as a result of serial tests of round unnotched specimens (fig. 3.6 a).

The point 1 determines the beginning of transition, and a temperature corresponding to this point is called the upper boundary of cold-brittleness threshold. This temperature is taken as cold-brittleness threshold and is denoted as . The point 2 corresponds to the end of ductile-brittle transition. The temperature at this point is called the lower boundary of cold-brittleness threshold.

While testing of notched specimens, especially specimens made of ductile materials (for example, low-carbon steels), the ductile-brittle transition takes place gradually in a wide temperature interval (fig. 3.6 б). At determination of for such materials some values of impact toughness are used. On the basis of these values a critical temperature is estimated.

 

 

Fig. 3.6. Serial curves of smooth (а) and notched (б) specimens test.

The following conditions are used in some cases: а _n = n а _n (where 0<n<1), usually n = 0,5 or n = 0,8. Critical temperatures obtained on the basis of these criteria are marked as and . The fixed values of impact toughness (equal to 2,5 kg×m/cm²) are also used. These fixed values are mostly used in west-european countries while testing specimens with the notch radius of 0,25 mm, its critical temperature is denoted as . The critical value of а _н = 3,0 kg×m/cm² is used in the Soviet practice, its critical temperature is denoted as .

Appendix 1
TASK

Variant	Steel №1	Steel №2	Impact toughness	Macroanalysis	Microfractography
1.	80С, h/r	20ХГ2Т, h/r	KCU	KCU	х240
2.	80С, h/r	25Г2С, h/r	KCU	KCU	х240
3.	80С, h/r	25Г2С, TS furnace	KCU	KCU	х240
4.	80С, h/r	25Г2С, TS mill	KCU	KCU	х240
5.	80С, h/r	20ХГ2Т, h/r	KCV	KCV	х1000
6.	80С, h/r	25Г2С, h/r	KCV	KCV	х1000
7.	80С, h/r	25Г2С, TS furnace	KCV	KCV	х1000
8.	80С, h/r	25Г2С, TS mill	KCV	KCV	х1000
9.	20ХГ2Т, h/r	25Г2С, h/r	KCU	KCU	х240
10.	20ХГ2Т, h/r	25Г2С, TS furnace	KCU	KCU	х240
11.	20ХГ2Т, h/r	25Г2С, TS mill	KCU	KCU	х240
12.	20ХГ2Т, h/r	25Г2С, h/r	KCV	KCV	х1000
13.	20ХГ2Т, h/r	25Г2С, TS furnace	KCV	KCV	х1000
14.	20ХГ2Т, h/r	25Г2С, TS mill	KCV	KCV	х1000
15.	25Г2С, h/r	25Г2С, TS furnace	KCU	KCU	х240
16.	25Г2С, h/r	25Г2С, TS mill	KCU	KCU	х240
17.	25Г2С, h/r	25Г2С, TS furnace	KCV	KCV	х1000
18.	25Г2С, h/r	25Г2С, TS mill	KCV	KCV	х1000
19.	25Г2С, TS furnace	25Г2С, TS mill	KCU	KCU	х240
20.	25Г2С, TS furnace	25Г2С, TS mill	KCV	KCV	х1000
21.	25Г2С, TS mill	80С, h/r	KCV	KCV	х1000
22.	25Г2С, TS furnace	20ХГ2Т, h/r	KCV	KCV	х1000
23.	25Г2С, TS mill	25Г2С, TS furnace	KCU	KCU	х240
24.	25Г2С, TS furnace	20ХГ2Т, h/r	KCU	KCU	х240
25.	25Г2С, TS mill	25Г2С, TS furnace	KCU	KCU	х240
26.	25Г2С, TS mill	80С, h/r	KCU	KCU	х240

Appendix 2
impact toughness data values after the test of impact specimens

KCU (type Ι)

Т, °С	80С, h/r	20ХГ2Т, h/r	25Г2С, h/r	25Г2С, TS furnace	25Г2С, TS mill
	0,28	0,92	1,54	1,42	2,08
	0,17	0,68	1,45	1,4	1,89
	0,09	0,63	1,15	1,35	1,06
	0,07	0,56	0,82	1,3	0,86
-20	0,06	0,52	0,63	1,17	0,68
-40	0,05	0,42	0,46	0,94	0,53
-60	0,05	0,22	0,3	0,75	0,43
-80	0,03	0,21	0,14	0,63	0,32
-100	0,03	0,2	0,1	0,57	0,23

 

KCV (type IV)

Т, °С	80С, h/r	20ХГ2Т, h/r	25Г2С, h/r	25Г2С, TS furnace	25Г2С, TS mill
	0,08	0,56	1,12	1,17	1,62
	0,06	0,3	0,83	1,18	1,6
	0,04	0,2	0,3	1,14	0,82
	0,06	0,17	0,22	1,13	0,3
-20	0,03	0,13	0,14	1,07	0,16
-40	0,02	0,08	0,11	0,82	0,13
-60	0,02	0,07	0,08	0,36	0,1
-80	0,02	0,05	0,04	0,27	0,05
-100	0,01	0,04	0,04	0,23	0,05

 

Appendix 3
macrophotographs of fractures

Test temperature: +100°С

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

Test temperature: + 60°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

 

Test temperature: + 20°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С h/r,
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

Test temperature: 0°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

 

Test temperature: - 20°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

Test temperature: - 40°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

Test temperature: - 60°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

Test temperature: - 80°С

 

Steel grade	KCV(type IV)	KCU(type I)
80С, h/r
25Г2С, h/r
25Г2С, TS furnace
25Г2С, TS mill

Test temperature: – 100°С

 

Steel grade	KCV(type IV)	KCU(type I)
25Г2С, TS furnace
25Г2С, TS mill

 

Appendix 4
microphotographs of fractures

Steel 80С, hot-rolled

t=+20°C, х240 t=+20°C, х240

 

 

t=+20°C, х1000 t=+20°C, х1000

 

 

Steel 20ХГ2Т, hot-rolled

 

t=+20°C, х240 t=+20°C, х240

 

 

t=+20°C, х1200 t=+20°C, х1000

 

Steel 20ХГ2Т, hot-rolled

 

 

t=+20°C, х1000 t=-20°C, х240

 

 

t=-20°C, х1000 t=-60°C, х240

 

 

Steel 20ХГ2Т, hot-rolled

 

 

t=-60°C, х240 t=-60°C, х1000

 

 

t=-60°C, х1000 t=-60°C, х1000

 

Steel 25Г2C, hot-rolled

 

t=+20°C, х240 t=+20°C, х240

 

t=+20°C, х1000 t=+20°C, х1000

 

 

Steel 25Г2C, hot-rolled

 

t=-20°C, х240 t=-20°C, х240

 

t=-20°C, х1000 t=-20°C, х1000

 

Steel 25Г2C, hot-rolled

t=-60°C, х240 t=-60°C, х1000

t=-60°C, х1000 t=-60°C, х1000

 

Steel of 25Г2C, TS furnace

 

 

t= +20°C, х240 t= +20°C, х240

 

 

t= +20°C, х1000 t= +20°C, х1000

 

 

Steel 25Г2C, TS mill

 

t= +20°C, х240 t= +20°C, х240

t= +20°C, х1000 t= +20°C, х1000

Steel 25Г2C, TS mill

 

t= -20°C, х240 t= -20°C, х240

 

t= -20°C, х1000 t= -20°C, х1000

 

Steel 25Г2C, TS mill

 

t= -60°C, х240 t= -60°C, х240

t= -60°C, х1000 t= -60°C, х1000

Appendix 5
design regulations of the course paper

The explanatory note to the course paper is conventionally subdivided into:

- introduction;

- main part;

- appendix.

Introduction

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- title page;

- task for the course paper;

- abstract;

- contents;

- list of conventional signs, symbols, contractions, terms.

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a) information about the ministry, higher educational establishment, department;

b) complete name of document. The division of words in the heading of title page is forbidden;

c) signatures of supervisors, tutors and the student who executed this the course paper. The signatures of responsible persons are placed in the following order: to the left there should be the white space for the personal signatures. To the right of this white space the posts, surnames, names and patronymics should be pointed in appropriate lines. Below the personal signatures, there should be pointed the dates of signing in Arabic numerals in the following sequence: year, month, date. For example, December, 26, 2002 should be designed as: 2002.12.26. or 02.12.26., verbally-digital dates design is also possible: December, 26, 2002.

d) year of explanatory note drawing up.

Task of the course paper

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There should be the topic of the course paper, data values, list of questions that should be studied in the task. The timetable of work stages preparation can also be shown in the task.

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