In t h e case o f ferritic st eel, t h e t yp e an d p ract icalit y o f h eat t reat m en t d ep en d h eavily o n t h e resp ect ive m at erial an d it s fo rm in g h ist o ry. O n
p rin cip le, eit h er o f t wo p o ssible p ro cesses can be u sed : recryst allizat io n an d n o rm al an n ealin g.
Gen erally, Mn o r Si allo ys are u sed fo r alum inium . Th e m o st favo rable m et h o d o f h eat t reat m en t m u st be d et erm in ed in each in d ivid u al case,
wh ereby p art icu lar at t en t io n m u st be p aid t o t h e lo n g-t erm beh avio r o f t h e m at erial, an d in t h e case o f st rain h ard en in g allo ys, t o rap id fu rt h er
p ro cessin g.
5.5.2 Preforms and preparation
Fo r h yd ro fo rm in g, p art s m ad e o f t h e fo llo win g sem i-fin ish ed p ro d u ct s are u sed
Fig. 5.2.8 :
– d rawn o r weld ed t u bes, – d o u ble-walled t u bes,
– ext ru d ed p ro files, – weld ed o r p re-fo rm ed t u bu lar blan ks.
In gen eral, t u bu lar p refo rm s wit h circu lar cro ss sect io n s are u sed . Weld ed an d ext ru d ed n o n -ro u n d sect io n s are u sed less freq u en t ly.
Th e p ro d u ct io n st ep s req u ired fo r t h e p rep arat io n o f p refo rm s fo r t ech n ically an d eco n o m ically o p t im u m p ro d u ct io n are d escribed in t h e
fo llo win g wit h referen ce t o t u bu lar blan ks. Th e t u be cutting p ro cess m u st be ext rem ely p recise in o rd er t o avo id
u n d esirable effect s su ch as leaks at t h e begin n in g o f t h e h yd ro fo rm in g seq u en ce. Th ese wo u ld lead t o failu re o f t h e co m p o n en t as a resu lt o f
bu cklin g. Flu ct u at io n s in t h e wall t h ickn ess alo n g t h e p erip h ery resu lt in a h igh scrap level, an d in ext rem e cases even t o failu re o f t h e co m -
p o n en t . Th e fo llo win g t o leran ces are gen erally ad m issible: – len gt h : + – 0.5 m m
– an gle o f t h e cro ss sect io n relat ive t o t h e lo n git u d in al axis: + – 0.5° Co n cen t ricit y at t h e t u be en d s is im p o rt an t in o rd er t o avo id u n wan t ed
sh earin g wh en t h e d ie is clo sed . Th e t u bes m u st be clean ed o f an y ch ip s, as t h ese lead t o t o o l d am age, fo rm in g erro rs o r m arks o n t h e su r-
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face o f t h e co m p o n en t . Tu be en d s m u st be d ebu rred fo r t h e sam e rea- so n , alt h o u gh ch am fers m u st be avo id ed u n d er an y circu m st an ces.
Th e id eal p refo rm fo r t h e h yd ro fo rm in g p ro cess is a st raigh t cylin d ri- cal t u be. Ho wever, d eviat io n s fro m t h is sh ap e are co m m o n , as t h e t u be
m u st be laid ad eq u at ely in t h e d ie in o rd er t o en su re p art q u alit y. Th is n ecessitates a pre-form ing p rocess. On p rin cip le, an y p re-form in g p rocess
red u ces su bseq u en t form ability an d lead s to d ifferen ces in th e wall th ickn ess an d fau lts in th e form in g beh avior of th e p arts as a resu lt of
local strain h ard en in g. However, su itable con figu ration of th e com p o- n en t an d op tim u m selection of th e p re-form in g p rocess can cou n teract
th ese n egative effects. Bending is p erfo rm ed o n a m an d rel, p ro file, cam o r in d u ct io n ben d -
in g m ach in e d ep en d in g o n t h e ben d in g rad iu s, t u be d iam et er an d wall t h ickn ess cf.
Sect . 4.8 . Becau se o f t h e n o t ch in g effect s t h at m ay o c-
cu r, su rface d am age, in p art icu lar sh arp -ed ged im p ressio n s creat ed by clam p in g jaws, m ay lead t o p rem at u re bu rst in g o f t h e co m p o n en t . Th e
t o leran ce fo r t h e st raigh t en d s, as fo r cu t t in g, is + – 0.5 m m . Ben d in g erro rs, su ch as wrin kles o n in sid e ben d s, im p ressio n s left by clam p in g
jaws o r d rawin g gro o ves m u st be avo id ed , p art icu larly in t h e case o f t h in -walled t u bu lar blan ks.
Reduction and expansion p ro cesses are u sed wh ere large p erip h eral d if- feren ces are req u ired o n lo n g elo n gat ed co m p o n en t s o r in case o f p ar-
t ial o versize o f t h e u sed t u be in t h e cen t ral sect io n . Here, t o o , p ro cess- sp ecific d efect s su ch as gro o ves, im p ressio n s et c. co u ld h ave a d et ri-
m en t al effect . Pre-form ing in the hydroform die rep resen t s an id eal so lu t io n if it can
elim in at e an y o t h er p re-fo rm in g st ep s req u ired o n t h e st raigh t t u be. Pre-fo rm in g is also carried o u t o n p art s wh ich h ave been p revio u sly
ben t , exp an d ed o r red u ced . Pre-form ing in the die is u sed in t h e case o f co m p lex geo m et ries, wh ere
t h e abo ve p ro cesses fail t o p ro d u ce sat isfact o ry resu lt s. Sp ecial p ro cesses also exist fo r t ap ered , weld ed t u bes, weld ed t u bes
wit h n o n -ro u n d sect io n s, d eep -d rawn h alf sh ells, ext ru d ed sect io n s an d jo in ed t u bes. In t h e case o f co m p lex co m p o n en t s, several p re-fo rm in g
st ep s can be n ecessary.
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5 Hydroforming
5.6 Presses for hydroforming
