FR2625227A1 - Alloy steel for nuts and bolts with checking of tightening using ultrasound - Google Patents

Abstract

This high-strength alloy steel for nuts and bolts with checking of tightening using ultrasound of the manganese-boron type comprises, in proportion by weight relative to the total weight, between 1.5 and 2.30 % of manganese, between 0.26 and 0.32 % of carbon, between 0.1 and 1.1 % of chromium, between 0.003 and 0.006 % of boron, between 0 and 0.2 % of nickel, between 0 and 0.2 % of molybdenum, between 0 and 0.1 % of copper, traces of other elements and iron as the remainder; in which formula the carbon, manganese, chromium, nickel and molybdenum components satisfy the following formula for thorough quenchability: 6 C + Mn + Cr + Ni + Mo >/= x, in which x is >/= 3.2 in the case of cold-forgeable screws and x is >/= 4.5 in the case of hot-forgeable screws.

Description

 "Alloy steel for bolts with ultrasonic tightening control".

 The screws and bolts are made of materials whose mechanical characteristics are determined according to the conditions of use of the bolt but also according to the manufacturing conditions.

To more easily determine. conditions of use, it is known to identify the materials by standardized classes defining the guaranteed minimum mechanical strength and yield strength which is guaranteed. For example, the current bolting, without heat treatment, is listed, according to the French standard NFE group 25000, in classes 6.8 corresponding to a mechanical resistance Rm of 600 N / mm 2 and to an elastic limit of 480 N / mm 2, while the High-strength bolts are listed in classes 10.9 or 12.9, corresponding to a mechanical resistance Rm of, respectively, 1000 and 1200 N / mm2 and at an elastic limit of 900 and 1080 respectively.
N / mm2.

 Traditionally, the high-strength bolting is, depending on the diameter of the screw, forged cold or hot and is subjected to a heat treatment consisting, essentially, quenching followed by income. To determine the mechanical characteristics of the material after quenching, characteristics varying according to the diameter of the screw, since it is easier to obtain hardening at the core for a small-diameter screw than for a large-diameter screw, for example greater at 22 mm, it is common to refer to the JOMINY index characterizing the guaranteed quenchability and deduced from a curve, of the type shown in FIG. 2, representing the variation of hardness HRC as a function of the distance between the measurement zone and the end face cooled by a jet of water, a test piece exiting quenching according to the standard of execution of the test.

 The characteristics of the material are also verified after recovery by means of a graph of the type shown in FIGS. 3 to 6, indicating, as a function of the tempering temperature, the mechanical strength Rm, the elongation A% and the elastic limit R 0.002. and KCU resilience as a result of measurements on standard specimens.

To obtain the desired mechanical characteristics, it is customary to use alloy steels, that is to say composed of iron and carbon to which various additives are added. Thus, the following alloy steels, designated according to the AFNOR standard, comprise the following additives, the proportions of which are indicated in average value.
35M5 1.35 0 of manganese, in addition to 0.25% of silicon and 0.36% of carbon,
32C4 1.05% chromium and 0.85% manganese, in addition to 0.25% silicon and 0.34% carbon,
34CD4 1.10. Chromium, 0.70% manganese, 0.25% mobybdenum in addition to 0.34% carbon and 0.25% silicon,
30NC11 2.80% nickel, 0.8% chromium, 0.50 t0 manganese in addition to 0.32 Ó carbon and 0.25% silicon.

 Due to the cost of additives, particularly chromium and nickel, alloy steel is much more expensive than standard steel, which limits its use. In addition, for certain additives, its metallurgical development is complex and also affects the final cost.

 One of the objects of the invention is to provide an alloy steel for high strength bolting which is less expensive than current alloy steels, can be cold or hot forged, and has mechanical performance that does not meet simultaneously. in the current standardized alloy steels.

 Furthermore, it is known to control the clamping stress of a connection by screw or bolt by means of an ultrasonic measuring device, for example according to the method described by the inventor in his French patent 2538470. To obtain a measurement reliable and repetitive, it turns out that the screw must not only be part of a batch from the same casting, machined in the same way and with reflection faces having precise positions and geometries, but also present in its constitutive material, after forging, tempering and returned a homogeneity of its crystalline structure so as to have, at constant temperature, a constant ultrasonic transparency index. The numerous tests carried out made it possible to note that the existing alloy steels, meeting the requirements of the high-strength bolts, did not show a good index of ultrasonic transparency, but, on the contrary, by the heterogeneity of their structure screams talline, disturbed the measurements.

 Another object of the invention is to provide an alloy steel for high strength bolts having a good ultrasonic transparency index, for performing reliable ultrasound checks.

 Finally, if we already know mangano-boreux alloys, they are not used in high strength bolting because their mechanical characteristics1 which vary according to the temperature of income, do not reach the optimal values at the temperatures of income recommended for bolts. In addition, alloy steels of this type are known in standardized compositions, leading to materials usable only for bolts of small diameters. They are not cold forging and do not provide the desired mechanical characteristics.

 The invention therefore aims to provide a manganous-borous alloy steel that is easy to develop, less expensive than alloy steel nickel-based or chromium-based, forgeable cold and hot, having better mechanical properties than those of alloy steels current, and having a good index of ultrasonic transparency.

This alloy steel comprises, in proportion to the total weight
- between 1.5 and 2.30 m of manganese,
- between 0.26 and 0.32 i Ó of carbon,
- between 0,1 and 1,1, Ó of chromium,
- between 0.003 and 0.006 Ó of boron,
between 0 and 0.2 Ó of nickel,
between 0 and 0.2 mol of mobybdenum,
between 0 and 0.1 'of copper,
- traces of phosphorus less than 0.025%,
- traces of sulfur less than 0,03 oX
traces of silicon less than 0.2 Ó,
- traces of titanium less than 0.005 bones
- traces of aluminum less than 0,04 m,
- and for the balance of the iron
formula in which the carbon, maganese, chromium, nickel and mobybdenum components satisfy the following hardenability formula
6C + Mn + Cr + Ni + Mo) x
where x is> 3.2 for cold forgiving screws and x is C to 4.5 for hot forging screws.

 The manganese-borous alloy steels of this family are characterized by a lowering of the carbon content to a value of less than 0.32, which is below the value of 0.35 characteristic of the iron-carbon diagram, and by a increase in maganese content of up to 2.30, o. This low-workability steel is also cold-forgeable, core-hardenable and has a homogeneous mechanical strength. Its hardenability and mechanical strength are improved by the presence of combined chromium, within the limits of the quenchability formula, with the carbon and manganese components and the traces of nickel and mobybdenum.

The mechanical characteristics of this family of alloy steels, after quenching, are the following mechanical strength Rm J 1250 N / mm2 (corresponding to the class 12.9 of the high strength bolting)
yield strength R 0.02 ± 0.9 Rm (class 12.9)
lengthening A 0) 14
fragility KCU 20) 8 daJ / cm2.

 Moreover, it has a good index of ultrasonic transparency guaranteeing the reliability of this method of measuring stresses in the nuts and bolts.

 The use of a manganese base, with a very small proportion of metals several times more expensive than it, such as chromium, nickel and mobybdenum, makes it possible to obtain an alloy that is much less expensive than the alloys of the same class used. currently for high strength bolting.

 It should also be added that alloy steels of this type are characterized by low hydrogenability, during the preparation and laying of protective coatings, a reduced fragility to the cold, and by a very low sensitivity to problems of superficial decarburization.

In a form of elaboration for the particular application to hot-forged bolts, this alloy steel comprises in proportion to the total weight
- from 1.80 to 2.30 nÓ of manganese,
from 0.29 to 0.32% of carbon,
from 0.4 to 0.8% of chromium,
from 0.003 to 0.006% boron.

from 0 to 0.2 Ó of nickel,
from 0 to 0.2 mol of mobybdenum,
from 0 to 0.1 Ó copper,
- traces of phosphorus, sulfur, silicon, titanium and aluminum,
- and for the balance of the iron
formula in which the components carbon, manganese, chromium, nickel and mobybdenum satisfy the hardenability formula at core 6C + Mn + Cr + Ni + Mo # 4,5
Thus, for example, a steel of this type corresponding to the AFNOR 30MCB8 definition comprises, by spectographic measurement carried out on a casting control sample, laminated, having undergone a globulization annealing, pickled, phosphated and drawn according to the process known under the name SKIN PASS
Mn 2%
C 0.30 Ó
Cr 0.6%
B 0.0035%
Neither 0.1 Ó
0.1 m
Traces of Cu, S, Si, Ti, Al
Iron on sale.

The hardenability formula:
6C + Mn + Cr + Ni + Mo> 4.5 gives
(6 X 0.3) + 2 + 0.6 + 0.1 + 0.1 = 4.6.

 In practice, an alloy steel of this type is intended for the manufacture of bolts having a diameter of between 22 and 48 mm, although it can also be used for smaller diameters.

 The bars, drawn to the appropriate diameter, are cut into pieces of suitable lengths. After heating between 850 and 9500C, these pieces are forged to form the head and body of the screw.

After machining and cold rolling of the net, the screw is subjected to heat treatments. These include quenching and income.

The quench is carried out by heating at 9000C + 100C, in a controlled atmosphere furnace, and by quenching in an oil tank at a temperature between 50 and 70 C. The income is carried out at a temperature of between 435 and 5500C and depending on the desired mechanical characteristics, and over a period equal to 1.5 fpis the austenization time, without being less than 45 minutes.

In a form of development, aimed at the application to the cold forgeable bolts, this alloy steel comprises, in proportion by weight relative to the total weight:
from 1.5 to 1.75% of manganese,
- from 0,26 to 0,29 Ó of carbon,
from 0.1 to 0.3% of chromium,
from 0.003 to 0.005% boron,
from 0 to 0.15, of nickel,
from 0 to 0.10 m of copper,
traces of mobybdenum, phosphorus, sulfur, silicon, titanium and aluminum,
- and for the balance of the iron,
formula in which the carbon, manganese, nickel, chromium and mobybdenum components satisfy the following core hardening formula 6C + Mn + Ni + Cr + Mo # 3,2,
while the nickel, chromium, copper and mobybdenum components satisfy the following forgeability formula
Ni + Cr + Cu + Mo <0.4.

For alloy steel of this type, corresponding to the definition
AFNOR 28MC86, the spectographic analysis carried out on a casting control sample, carried out under the same conditions as for the 30MCB8, indicates
- Mn 1.63
- C 0.27
- B 0.0032
- Ni 0.08
- Cr 0.17
- Cu 0.08
- 0.01 MB
- traces of Cu, S, Si, Ti and Al
- and Fe on sale.

The hardenability formula:
6C + Mn + Cr + Ni + Mi) 3.2 gives
(0.27 X 6) + 1.63 + 0.17 + 0.08 + 0.01 = 3.51,
while the forgeability formula
Ni + Cr + Cu + Mo 4 0.4donne
0.08 + 0.17 + 0.08 + 0.01 = 0.34.

 In practice, an alloy steel of this type is intended for the cold-forged manufacture of high-strength bolts having a diameter of less than 22 mm.

 The wire, stretched according to the SKIN PASS method and provides the rolling diameter, feeds a multi-station press performing cold forging of the head. After chamfering the barrel, also called pointing, the net is made by cold rolling.

 The screws obtained are then subjected to the heat treatments whose conditions are specified above with respect to the alloy steel 30MCB8.

 The characteristics of this family of steel will now be compared with those of particular standardized alloy steels, with reference to the curves shown in the appended figures.

Figure 1 is a graph showing the variation of tensile force F, in Newton, as a function of the elongation A (in o) of a standard tensile test piece stretched on an AMSLER press and giving the shape of the curve of elongation of an alloy steel according to the invention,
Figure 2 is a graph showing the variation in hardness
HRc as a function of the distance, in mm, between the measuring zone and the cooling zone of the specimen, corresponding to the radius of the specimen, and showing the hardenability curves or curves
JOMINY steels according to the invention compared to reference alloy steels,
Figures 3 to 6 are graphs indicating, as a function of the tempering temperature, given in abscissa, on the ordinate on the left, the mechanical strength Rm and the elastic limit R 0,02 in N / mm2 and, on the ordinate on the right, 'percentage elongation and fragility
KCu in daJ / cm2; and this by comparing the characteristic curves of the alloy steels according to the invention with those of reference alloy steels defined according to the AFNOR standard.

 Figure 7 is a graph indicating, based on the temperature of the income, and thus the characteristics of the classes of bolts, the KCU resilience in daJ / cm2 of a test piece of 28MCB6 for different temperature tests CHARPY sheep.

 In FIG. 1, showing the shape of the tensile curve of the alloy steels according to the invention, it can be seen that the angle α 'characterizing the modulus of elasticity is important, and that the transition dt between the phase of elastic deformation of, characterized by the line OE, and the phase of permanent deformation dp, considered conventionally from the point P corresponding to R 0.002, is carried out according to a regular radius indicating a gradual phenomenon.

 It should be remembered that this zone is often characterized by a plateau or undulations reflecting an intercrystalline decohesion, which does not appear here.

 In addition, on the curve of Figure 1, the zone of permanent deformation dp is represented by an arc of curve of great radius and great length, justifying the elongation of 14, Ó before necking and the great guarantee provided by this alloy steel .

 In FIG. 2, the curves A1 and A2 corresponding to the limits of the characteristics of the alloy steels according to the invention and respectively of the 28MCB6 and the 30MCB8.

 Examining this graph, it can be seen that for distances ranging from 0 to 10 and 25 mm respectively from the left-hand ordinate, curves A1 and A2 are slightly inclined on the horizontal, which demonstrates the excellent hardenability. at heart for screws of diameter from 0 to 20 for Al and from 0 to 50 for A2. In these parts of the curves, the measured hardness is higher than those of the reference steels for larger diameters. As a result, such alloy steels satisfy, after income, the conditions of the bolting class 12.9 shown on the straight line T, for screws of larger diameters than the maximum ones attained by the steels of reference defined in the preamble and whose As shown in curve A3, representative of 35M5 alloy steel, chosen because of the absence of boron and chromium in its composition, the latter can not be used for screws having a diameter greater than 10 mm, even in class 10.9, represented by line S.

 The curve A4, corresponding to a cold-forging alloy steel 32C4 and allowing the production of hexagonal screws, shows that this material has an improved JOMINY index, but lower than that of the alloy steels according to the invention.

 Finally, the curve A5, corresponding to a 34CD4, usable in hot forging to make screws having a diameter less than or equal to 22 mm, has better characteristics, but these remain however lower than those of the steels according to the invention. .

Its composition is indicated in the preamble.

 In addition, and this is a significant disadvantage, the three reference steels 35M5, 32C4 and 34CD4 are not usable for stress measurements by ultrasound or ultrasonic elasticity, because their crystal structure causes ultrasonic inaccuracies.

 It is not the same for alloy steels according to the invention which, not only is a good index of ultrasonic transparency, but in addition have superior mechanical performance as confirmed by the following curves of FIGS. curves represent the variations of the mechanical characteristics of the family of alloy steels according to the invention, as a function of the variations in the tempering temperature between 400 and 6000 C, and by comparison with reference alloy steels. In other words in these FIGS. 3 to 6, the curves B1, B2, B3, B4 represent the variations respectively of the mechanical strength Rm, the elastic limit R 0.02, the elongation A and the resistance KCU for an alloy steel according to the invention having a composition corresponds to the AFNOR 28MCB6 definition.

In FIG. 3 and in comparison with the curves C1, C2, C3,
C4 corresponding to B1, 82, 83,84, but relating to the characteristics of a 30NCD16 steel containing, inter alia, 0.30% carbon, 4% nickel, 1.37, Ó chromium and 0.5 D of mobybdenum and known for its very high performance, it is found that the 28MC86 is superior in elongation A 50 and KCU resilience and that its mechanical strength Rm and elastic limit are slightly lower. This makes the alloyed steel according to the invention preferred to 30NCD16 which is more expensive and requires expensive forming treatments, such as intermediate annealing at a slow speed.

 In FIG. 4, the curves D1, D2, D3 and D4 indicate the characteristics of a 30NC11 alloy steel comprising 0.30% carbon, 2.7% nickel and 0.8% chromium. This material, which is not cold-forgeable, is expensive and has good quenchability, and is known to be the least fragile of alloyed heat treatment steels. However, when comparing its curves with those B1 to 84, we see that for mechanical strengths hardly inferior, the 28MCB6 has a very much greater elongation and resilience.

 These good results of the 28MCB6 are furthermore confirmed by the graph of FIG. 7 showing that, for a test temperature of 200C, the curve L1 of the 28MC86 is well above the curve M of a 30NC11, so that its curves L2 and L3 correspond to tests carried out at low temperature, respectively at -400C and -600C, are well above those of this alloy and known alloy steels.

This shows that alloy steel 28MC86 and the steel family according to the invention retain a higher resilience than the best existing steels at low temperatures.

In Figure 5, the comparison is made with the curves
E1 to E4 of a 34CD4 steel, comprising 0.34, carbon, 1.1, chromium and 0.25 mobybdenum, already mentioned in the introduction as being expensive and having poor ultrasonic characteristics.

 The 28MCB6 has similar mechanical characteristics but superior elongation and resilience.

 Finally, FIG. 6 compares with curves F1 to F4 a 35NC6 comprising 0.33% carbon, 3.5% nickel and 1% chromium. It is a very expensive alloy steel, not cold forgeable, called safety, because commonly used in aeronautics because of its mechanical performance. For similar mechanical strengths, the 28MCB6 is also very much superior in resilience and elongation.

 The family of alloy steels according to the invention differs from existing alloy steels by good forgeability, cold or hot, by good hardenability at heart, resulting in better mechanical characteristics, satisfying the highest classes of high-grade bolts. resistance, and a homogeneous crystalline structure providing reliability to ultrasonic testing and measurements at a much lower cost of composition and elaboration.

 Compared to existing manganese-borous steels having standardized compositions of the 20MB6, 25MB5 or 38MB5 type, the alloyed steels according to the invention, of the 28MCB6 or 30MCB8 type, have in addition and simultaneously the following characteristics: oil hardenability, hot or cold forgeability according to the index of the forgeability formula, better JOMINY index and better mechanical characteristics for tempering temperatures between 4350C and 5500C.

 Of course, these characteristics are combined simultaneously with excellent ultrasonic transparency, insensitivity to superficial decarburization, and virtually insensitivity to hydrogenability during surface treatment operations.

Claims (3)

 1. Alloy steel for high-strength bolts with ultrasonic clamping control type manego-boreux characterized in that it comprises, in proportion by weight relative to the total weight  between 1.5 and 2.30 Ó of manganese,  - between 0,26 and 0,32 Ó of carbon,  - between 0.1 and 1.1 of chromium,  between 0.003 and 0.006% of boron,  between 0 and 0.2% of nickel,  between 0 and 0.2% of mobybdenum,  between 0 and 0.1% of copper,  - traces of phosphorus less than 0,025, ÓS  traces of sulfur less than 0.03,  traces of silicon less than 0.2 0,  - traces of titanium less than 0.005 o  - traces of aluminum less than 0,04 0  - and for the balance, iron  formula in which the carbon, manganese, chromium, nickel and mobybdenum components satisfy the following hardenability formula  6 C + Mn + Cr + Ni + Mo? x  where x is at 3.2 for cold-forceps and x is at 4.5 for hot-forgeable screws.  2. Alloy steel according to claim 1, characterized in that, in the case of its application to hot forging screws it comprises, in proportion by weight relative to the total weight  from 1.80 to 2.30% of manganese,  from 0.29 to 0.32% of carbon,  from 0.4 to 0.8% of chromium,  from 0.003 to 0.006 Ó of boron,  from 0 to 0.2 mol of nickel,  from 0 to 0.2% of mobybdenum,  from 0 to 0.1% copper,  - traces of phosphorus, sulfur, silicon, titanium and aluminum,  - and for balance, iron,  formula in which the components carbon, manganese, chromium, nickel and mobybdenum satisfy the following quenchability formula  6C + Mn + Cr + Ni + MO 4.5.  3. Alloy steel according to claim 1 characterized in that, in the case of its application to cold forging screws, it comprises, in proportion by weight relative to the total weight,  from 1.5 to 1.75 g of manganese,  from 0.26 to 0.29 m of carbon,  from 0.1 to 0.3% of chromium,  from 0.003 to 0.005% of boron,  from 0 to 0.15% of nickel,  from 0 to 0.10 g of copper,  traces of mobybdenum, phosphorus, sulfur, silicon, titanium and aluminum,  - and for the balance, iron,  the components carbon, manganese, nickel, chromium and mobybdenum satisfy the following hardenability formula 6C + Mn + Ni + Cr + MO # 3,2,  while the manganese, nickel, chromium and mobybdenum components satisfy the following forgeability formula  Ni + Cr + CU + Mo # 0.4.