EAST AI Hydraulic Manufacturer
Since 2006
Views: 0 Author: ALEX Publish Time: 2026-09-15 Origin: Site
Written by Alex, Production Engineer at EAST AI
A cylinder builder called us not long ago frustrated that two batches of tube he considered equivalent were behaving completely differently on his welding line. One batch welded cleanly at the end caps and passed pressure test first time, while the other was cracking at the weld and driving up his rework rate. When we looked at what he had actually purchased, the answer was immediate: one batch was ST52 honed tube and the other was CK45, and although both are excellent cylinder tube materials, they are not substitutes for one another the moment welding enters the picture.
This is one of the most common material misunderstandings we see, and it is worth clearing up properly because the choice between ST52 and CK45 affects far more than the price on the quotation. The two steels sit in different metallurgical families, they respond differently to welding and machining, and they carry different strength and wear characteristics into service. Choosing correctly means matching the steel to how the cylinder is manufactured and how it will be loaded, rather than treating honed tube as a single commodity that happens to come in two grade names.
ST52, more precisely designated E355 or St 52-3 in the older German nomenclature, is a low-carbon structural steel with a carbon content typically around 0.20 percent or below. That relatively low carbon level is exactly what makes it forgiving to weld, because there is not enough carbon in the matrix to form the brittle, crack-prone microstructures that appear at a weld heat-affected zone in higher-carbon steels. ST52 is a fine-grained structural steel valued for good weldability, solid yield strength, and consistent behavior, which is why it is the default choice for welded cylinder constructions where end caps and mounting features are joined to the tube.
CK45, known also as C45 or in the American system as SAE 1045, is a medium-carbon steel with carbon content around 0.45 percent, and that higher carbon is the whole point of it. The additional carbon gives CK45 substantially higher strength and hardness in the normalized or quenched-and-tempered condition, and it allows the material to be surface hardened where wear resistance matters. The trade-off is that the same carbon that delivers strength also makes CK45 far more sensitive to welding, since the heat-affected zone can harden and crack unless preheating and controlled cooling are carefully applied.
For the large population of welded hydraulic cylinders built with welded end caps, welded ports, and welded mounting eyes, ST52 is almost always the right honed tube choice, and the reason is purely practical. Its low carbon content means a fabricator can weld the tube with standard procedures, without elaborate preheat regimes, and still achieve sound, crack-free joints that survive pressure testing and cyclic service. That reliability on the shop floor is worth a great deal, because weld defects are expensive to find, expensive to repair, and dangerous if they escape into the field.
CK45 can be welded, but it demands respect and process control that many general fabrication shops are not set up to deliver consistently. Preheating to reduce the cooling rate, careful control of heat input, and sometimes post-weld heat treatment are needed to avoid a hard, brittle heat-affected zone, and skipping those steps is exactly what produced the cracking our frustrated customer experienced. When a design genuinely requires CK45 strength and also requires welding, the manufacturing route has to be planned around that combination from the start rather than discovered during production.
None of this means CK45 is the lesser material, because in the right application its higher strength is precisely what the design needs. Tie-rod cylinders, where the tube is clamped between end caps rather than welded to them, sidestep the weldability problem entirely and let the designer take full advantage of CK45's superior mechanical properties. In those constructions the higher yield strength can allow a thinner wall for a given pressure rating, or a higher pressure rating for a given wall, and the surface can be hardened where the piston bears against the bore.
CK45 also becomes attractive wherever the bore itself must resist wear or the cylinder runs at pressures that push a lower-strength steel toward its limits. Because the medium-carbon composition responds well to heat treatment, it offers a path to a harder, more wear-resistant bore surface that ST52 simply cannot match in the as-supplied condition. The engineering question is always whether the application can accommodate CK45's manufacturing demands in exchange for those mechanical gains, and for non-welded, high-strength designs the answer is frequently yes.
Both grades machine and hone well when supplied correctly, and both can be finished to the tolerances hydraulic seals require, which is why the surface finish specification alone will not tell you which steel you are holding. We routinely finish honed tube in either grade to H8 or H9 bore tolerance with surface roughness in the Ra 0.4 to 0.8 micron range, and both steels take the characteristic honed cross-hatch pattern that retains lubricant and supports the seal package. The tolerance and finish are a function of the honing process, not of the base steel.
Where the steels differ in the machine shop is in how they respond to cutting and subsequent operations. CK45's higher carbon gives it different chip formation and, in the hardened condition, greater tool wear, so downstream operations such as boring for ports or machining mounting features need to be planned with the harder material in mind. ST52's lower strength makes it more straightforward to machine and to weld in the same production flow, which is another reason it dominates welded cylinder production where multiple secondary operations follow the tube itself.
The clean way to decide is to start with construction method and load, not with grade preference or unit price. If the cylinder is welded, ST52 should be the default and the burden of proof sits with anyone proposing CK45, because the welding risk is real and the consequences of a bad weld are severe. If the cylinder is a tie-rod or otherwise non-welded design that needs maximum strength or a wear-resistant bore, CK45 rewards the choice and its manufacturing demands are far easier to satisfy without weld joints in the assembly.
In the case that opened this article, the fix was simply to standardize the welded product line on ST52 and reserve CK45 for a separate high-pressure tie-rod model, matching each steel to the construction it suited. The welding cracks disappeared, the rework rate fell back to normal, and the customer stopped treating the two grades as interchangeable line items. Getting that division right at the specification stage is far cheaper than discovering it through failed pressure tests, and it is exactly the kind of decision we help customers make before the first tube is cut.
ISO 286-1 — ISO code system for tolerances on linear sizes (basis for the H8/H9 bore tolerance grades cited above): https://www.iso.org/standard/45975.html
Specifying ST52 when you needed CK45, or welding a grade that was never meant to be welded, is a mistake that stays hidden until pressure test day or, worse, until the field. At EAST AI we have supplied both grades into welded and tie-rod cylinder production since 2006, and we will help you match the steel to your construction method and load case before a single length is cut, so the decision is made on engineering rather than on guesswork.
What We Offer
ST52 (E355) honed tube optimized for welded cylinder construction and reliable weld joints
CK45 (SAE 1045) honed tube for high-strength tie-rod designs and wear-resistant bores
H8 / H9 bore tolerance with Ra 0.4–0.8 μm controlled surface finish in both grades
Grade selection guidance tied to your construction method, pressure rating and duty cycle
