Automotive Leaf Spring

Abstract

Automotive leaf springs are produced from low-hardenability and specified hardenability steel, with identical and different length, width and thickness and constant or variable cross section profile, that are subjected to through-surface hardening and low tempering. The ideal critical diameter of hardening, carbon content and hardened layer depth depend on the thickness of constant cross section profile leaf and maximum and minimum thicknesses of variable cross section profile leafs. Adherence to the optimum correlations of parameters indicated make it possible to produce leaf springs with the highest mechanical properties and longevity.

Claims

1. An automotive leaf spring that consists of one or several leafs produced from low hardenability (LH) and specified (SH) steel, having identical or different lengths, widths and thicknesses, with constant or variable cross section profile, that were subjected to through-surface hardening (TSH) and low tempering, with a distinction that the leafs are made from low hardenability (LH) and specified hardenability (SH) steel with 0.2-0.8% carbon content, as follows: for constant profile leafs: the ideal critical hardening diameter (C.sub.cr., mm) has the following values, depending on the leaf thickness (H, mm): D.sub.cr=(0.6-1.2) H, mm, which ensures the harden layer depth (mm) equal to =(0.1-0.22) H; for variable profile leafs: depending on the leaf thickness variation from minimum (h.sub.0, mmat the end support points) to maximum (H.sub.0, mmin the leaf center), the range of permissible leaf cross section profiles (h.sub.0/H.sub.0) should not be outside the limits constrained by zones I, II, III on the graph that shows relative thickness h/H.sub.0 versus its relative length//L.sub.0 (see graph); herewith: for zones I-II, where h.sub.0/H.sub.0=0.45-0.55, the LH (SH) steel ideal critical hardening diameter D.sub.cr.=(0.95-1.2) h.sub.0=(0.45-0.65) H.sub.0, which ensures the harden layer depth along the leaf length l from =(0.15-0.22) h.sub.0 at the minimum leaf end thickness to =(0.07-0.125) H.sub.0 at the maximum leaf thickness in its mid-section; for zone III, where h.sub.0/H.sub.0=0.55-0.65, the LH (SH) steel ideal critical hardening diameter D.sub.cr.=(0.95-1.2) h.sub.0=(0.55-0.75) H.sub.0, which ensure the harden layer depth along the leaf length l from =(0.15-0.22) h.sub.0 at the nominal leaf end thickness to =(0.1-0.145) H.sub.0 at the maximum leaf thickness in its mid-section.

2. An automotive leaf spring according to claim 1, in which working surfaces of each constant cross section profile leaf with thicknesses bigger than 8 mm and variable profile leaf with a thickness of 0.45-0.65 of the maximum central thickness, but not less than 8 mm, are made from LH (SH) steel with 0.4-0.8% carbon content and have a martensite structure with ##1-5 acicularity, 50 . . . 62 HRC hardness and depth equal to 0.07-0.22 of the leaf thickness, the core structure being troostite, troosto-sorbite-sorbite with 30-50 HRC hardness and #10-14 actual austenite grain.

3. An automotive leaf spring according to claim 1, the working surfaces of each constant cross section profile leaf with thicknesses bigger than 8 MM and variable profile leaf with 0.45-0.65 of the maximum central thickness, but not less than 8 mm, are made from LH (SH) steel with 0.4-0.8% carbon content and have a hardened textured martensite structure with ##1-5 a cicularity, 50 . . . 62 HRC hardness and depth equal to less than 0.22 of the leaf thickness, plastically deformed surface, the core structure being troostite, troosto-sorbite-sorbite with 30-50 HRC hardness and #10-14 actual austenite grain.

4. An automotive leaf spring according to claim 1, spring leaf thin end sections with variable cross section profile (thickness less than 8 mm) and constant profile (less than 8 mm) may be produced with through hardening to form a tempered martensite or tempered martensite in the surface layer with 45-60 HRC hardness, the core structure being troosto-martensite with ##1-5 acicularity and #10-14 actual austenite grain, with 0.2-0.4% carbon content in the LH steel.

5. An automotive leaf spring according to claim 1, spring leaf thin end sections with the variable cross section profile (thickness less than 8 mm) and constant profile (less than 8 mm) may be produced with through hardening to form tempered martensite in the surface layer, the core structure being troosto-martensite, with the surface layer of not more than 0.22 of the leaf thickness, martensite is a cold-worked texture of the plastically deformed surface per claim 3.

6. An automotive leaf spring according to claim 1, that in the vicinity of the spring leaf central hole or centering indentation surface there is a zone with martensite, troosto-martensite, troostite, troosto-sorbite, hardening soorbite structure or with additional <50 mm local tempering from 2 sides, including the hole surface, with 30-56 HRC hardness.

7. An automotive leaf spring according to claim 1, that the spring leaf working surfaces exposed to tensile stresses caused by external forces during operation are subjected to pre-machining that results in formation of a not fully decarburized, <0.1 mm deep, layer.

8. An automotive leaf spring according to claim 1, TSH of spring leafs made from steels per claim 1 is combined with thermochemical treatment (TCT)-carburization or high temperature carbonitriding (CN) after the initial or repeated heating from a lower optimal temperature that provides for ##10-14 austenitic grain.

9. An automotive leaf spring according to claim 1, the working surfaces of each constant and variable cross section profile leaf with thicknesses bigger than 5 mm, after TCT and TSH, have a carburized martensite structure with carbon content not exceeding 0.8%, but not less than 0.15% higher than in the initial core with 45 . . . 62 HRC hardness and depth that exceeds the initial decarburized layer produced during rolling, less than 0.22 of the leaf thickness; wherein the microstructure and total hardening depth after TCT and TSH are in accordance with claims 2-5; in the corein accordance with claims 2-5.

10. An automotive leaf spring according to claim 1, with a distinction that the working surfaces of each constant and variable cross section profile leaf with thicknesses bigger than 5 mm, after TCT and TSH, have a carburized hardened martensite structure with carbon content not exceeding 0.8%, but not less than 0.15% of its content in the initial core with 45 . . . 62 HRC hardness and depth that is less than 0.22 of the thickness of the leaf produced after shot blasting of the surface; the core microsection is in accordance with claims 2-5.

11. An automotive leaf spring according to claim 1, spring leafs with the carbon content of higher than 0.6% in the steel or in the surface layer are, after hardening, subjected to treatment with cold at temperatures not higher than minus 60 C.

Description

[0016] This invention contain specific limiting values of the LH and SH steel ideal critical diameter (DI.sub.cr.) and carbon content depending on the thickness of constant and variable profile spring leafs. Non-availability of these main characteristics is the shortcoming of the known published materials.

[0017] For example, based on thermophysical calculations for leafs with a constant cross section profile and thickness H (mm), DI.sub.cr, min. is 0.6 H, mm, but not less than 6.0 mm, while DI.sub.cr max is 1.2 H, mm, i.e. DI.sub.cr. Is (0.6-1.2) H, mm, which ensures the =(0.1-0.22) H harden layer depth. Table 1 shows permissible DI.sub.cr. Values of the harden layer depth with respect to the constant-profile leaf thickness H. The carbon content in steel is 0.4-0.8%.

TABLE-US-00001 TABLE 1 H, mm 8 9 10 12 14 16 18 20 22 24 30 40 45 50 DI.sub.cr, 6-10 6-11 6-12 7-14 8-16 10-19 11-21 12-24 13-26 14-28 18-36 24-48 27-54 30-60 mm , mm 0.8-1.7 0.9-2.0 1.0-2.2 1.2-2.6 1.4-3.1 1.6-3.5 1.8-4.0 2.0-4.4 2.2-4.8 2.4-5.3 3.0-6.6 4.0-8.8 4.5-10.0 5.0-11.0

[0018] The ideal critical hardening diameter for constant cross section profile leafs whose thickness H is less than 8 mm is: DI.sub.cr.>6 MM, C=0.2-0.4%.

[0019] Various optimal designs have been developed for variable cross section profile leafs that are eligible for TSH.

[0020] FIG. 1 shows the loading diagram of a spring leaf half with a fixed central section.

[0021] In this case, the maximum bending stress is .sub.bend.=6Pl/bh.sup.2=const, where:

[0022] P is the support reaction (const);

[0023] / is the length of the arm of force P;

[0024] h is a variable value equal to the leaf thickness over length l;

[0025] L.sub.0 is the distance from the leaf end (end reaction point) to the fixed-end (const);

[0026] b is the leaf width (const); H.sub.0, h.sub.0 are the biggest and smallest leaf thicknesses (const).

[0027] By substituting L.sub.0 for l and H.sub.0 for h in the formula, we get:

.sub.bend.=6PL.sub.0/bH.sub.0.sup.2=const;

[0028] By equating two expressions, we get h/H.sub.0=(l/L.sub.0)

[0029] FIG. 2 contains a graph that shows relative leaf thickness h/H.sub.0 versus the relative length l/L.sub.0. The graph is a theoretical contour of a constant cross section leafthe dot-and-dash curve.

[0030] There are five zones here, three of which are confined to straight lines: [0031] 0zone with overloaded thin end leaf sections compared with thicker, underloaded mid-sections; [0032] Iminimum TSH application zoneh.sub.0H.sub.0=0.45 min; [0033] IImost optimal TSH application zoneh.sub.0/H.sub.0=0.45-0.55; [0034] IIITSH-acceptable zone which, however, is a high structural rigidity zoneh.sub.0/H.sub.0=0.55-0.65; [0035] IVirrational configuration zone, where the leaf mass, rigidity and load non-uniformity during bending along it length grow sharply; it is low at the end zones and maximum in the central zone.

[0036] With h.sub.0/H.sub.0>0.65, the TSH method may practically be applicable, and, as it approaches h.sub.0/H.sub.0=1, the spring leaf will have a constant profile.

[0037] Thus, in case of variable profile leafs, minimum thickness h.sub.0 is the limiting parameter for thermophysical calculations.

[0038] With DI.sub.cr, min.=0.95 h.sub.0 (mm), DI.sub.cr, max=1.2 h.sub.0, the harden layer depth is:

[0039] For zones III: h.sub.0=(0.45-0.55) H.sub.0, =(0.15-0.22) h.sub.0 or =(0.07-0.12) H.sub.0, DI.sub.cr,=(0.95-1.2) h.sub.0=(0.4-0.65) H.sub.0

[0040] For zones III: h.sub.0=(0.55-0.65) H.sub.0, =(0.15-0.22) h.sub.0 or =(0.08-0.145) H.sub.0, DI.sub.cr,=(0.95-1.2) h.sub.0=(0.5-0.75) H.sub.0

[0041] Table 2 shows permissible ideal critical hardening diameters DI.sub.cr, of steel with 0.4-0.8% C depending on the thickness variationfrom minimum h.sub.0(mm) to maximum H.sub.0 (mm) with the most optimal variable cross section profile ratio h.sub.0/H.sub.0=0.5.

TABLE-US-00002 TABLE 2 h0/H0 = 0.5 8/16 9/18 10/20 11/22 12/24 14/28 16/32 18/36 20/40 25/50 DI.sub.cr, mm 7-10 8-11 9-12 10-14 11-15 13-17 15-20 17-22 18-24 23-30 , mm 1.2-1.8 1.3-2.0 1.5-2.2 1.6-2.4 1.8-2.6 2.1-3.1 2.4-3.5 2.7-3.9 3.0-4.4 3.7-5.5

[0042] Therefore, the TSH method is applicable for hardening of spring leafs thicker than 8 mm.

[0043] For variable profile leafs, with the minimum thickness of less than 8 mm and maximum more than 8 mm, i.e. h.sub.0<8 mm, H.sub.0=(8-16) mm, DI.sub.cr,=(6-10) mm, C=0.2-0.4%.

[0044] An important distinctive feature of this invention is the specific dependence of two main LH (SH) steel parameters, i.e. its carbon content (% C) and ideal critical diameter DI.sub.cr., on the leaf thickness H(h.sub.0). In this case, the permissible nominal carbon content of a specific steel with narrow tolerances (0.05% or 0.025%) is selected from broad limits (0.2-0.8% C). For constant and variable cross section profile leafs, permissible ideal critical diameter DI.sub.cr, values are also selected from broad limits(6-60 mm) and 7-30 mm), respectively. Within the framework of Russian Federation patents #2450060, bul. #13, Oct. 5, 2012; and #2450079, bul. #13, Oct. 5, 2012, with the same DI.sub.cr, steel chemical elements other than carbon have no impact.

[0045] A qualitatively low solution with regard to thermal strengthening of constant and variable cross section spring leafs is a combination of TSH and thermochemical treatment (TCT)carburization (C) or high temperature carbonitriding (CN), that makes it possible to:practically completely eliminate spring leaf surface decarburization to the matrix that had occurred during rolling; carbon content in steel is 0.2-0.4%; in this case, the maximum carbon content in the martensite-structure surface layer should not exceed 0.8%, while the minimum carbon content should be more than 0.2% higher than that in the matrix that has a martensite, troosto-martensite, troostite structure (depending on the leaf thickness); [0046] reduce the minimum thicknesses of constant profile spring leafs subjected to thermal treatment from 8 mm (TSH) to 5 mm (TCT), and that of variable profile leafs from 8/16 mm (TSH) to 5/10 mm(TSH+TCT); [0047] improve the leaf spring durability by additionally creating beneficial residual compressive stresses in the leaf surface layer that were formed as a result of TCT; [0048] work harden the spring leaf surface from its stressed side, during operation, that was effected by shot peening or another mechanical method resulting in higher fatigue endurance similarly to what was described above.